Estimates Of Natural Mortality Research Articles

Puzzling parrotfishes: Radiocarbon age validation and updated longevity estimates for western Atlantic species in support of sustainable fisheries management

For management efforts to succeed in Caribbean fisheries, local fishers must support and be willing to comply with fishing regulations. This is more likely when fishers are included in a stock assessment process that utilizes robust scientific evidence, collected in collaboration with fishers, to evaluate the health of fish stocks. Caribbean parrotfishes are important contributors to coral reef ecosystem health while also contributing to local fisheries. Scientifically robust stock assessments require regional species-specific information on age-based key life history parameters, derived from fish age estimates. Evaluation of the accuracy of age estimation methods for fish species is a critical initial step in managing species for long-term sustainable harvest. The current study resulted from a collaborative research program between fish biologists and local fishers investigating age, growth, and reproductive biology of the seven parrotfish species landed in U.S. Caribbean fisheries; specifically, we validated age estimation for stoplight parrotfish Sparisoma viride and queen parrotfish Scarus vetula. This is the first study to directly validate age estimation for any parrotfish species through analysis of Δ14C from eye lens cores. Our age estimation validation results show that enumeration of opaque zones from thin sections of sagittal otoliths for a Sparisoma and a Scarus species provides accurate age estimates. The oldest stoplight parrotfish and queen parrotfish in the Δ14C age estimation validation series were 14 y and 16 y; while the oldest stoplight parrotfish and queen parrotfish we aged to-date using the Δ14C validated age estimation method were 20 y and 21 y, respectively. Fish longevity (maximum age attained/life span) is a key life history parameter used for estimation of natural mortality, survivorship, and lifetime reproductive output. Past reviews on parrotfishes from the Pacific and Atlantic concluded that most Caribbean/western Atlantic parrotfish species are relatively short-lived with estimated maximum ages ranging from 3–9 y. However, information from our collaborative research in the U.S. Caribbean combined with recently published age estimates for Brazilian parrotfish species indicate that many western Atlantic parrotfishes are relatively long-lived with several species attaining maximum ages in excess of 20 y.

Natural mortality-at-age estimations for the benthonic commercial pen shell Atrina maura

Natural mortality-at-age (Mt) is key for fisheries stock management, especially on sedentary resources such as bivalves under heavy adult and juvenile exploitation. This study aims to calculate Mt for the commercial bivalve Atrina maura from the Gulf of California by applying two approaches: the gnomonic interval model (GIM) and Chen & Watanabe model. The GIM uses biological data to assemble Atrina's life cycle into gnomonic intervals. This study also explores the effect of the variability in embryonic development under short and long-longevity scenarios on Mt. The Chen and Watanabe model uses the growth parameters of the species found in the literature to estimate Mt. Results show that both models described the typical form of Mt with high mortality rates at early ages and a reduction and stabilization of Mt from the age of first maturity to adulthood. The range of M values based on the GIM was 2084 (larval development) to 0.71 yr−1 (adults) for the 6 years longevity scenario and 2084 (larval development) to 0.39 yr−1 (adults) for the 10 years longevity scenario. For the Chen and Watanabe model Mt ranged from 4.6 (juveniles) to 0.73 yr−1 (adults). The Mt vectors at early ages showed higher values in the GIM (8.0 vs. 4.6 yr−1), followed by similar values during the age of sexual maturity (0.59 yr−1 for both models), and slightly high values in the Chen Watanabe model at older ages (0.39 vs. 0.64 yr−1). Furthermore, the estimates of natural mortality based on independent studies for larvae and adults of A. maura showed similar values with the Mt vectors estimated in this study, confirming the validity of their application on bivalve stock assessment by reducing bias and improving bivalve fisheries management.

Length-based assessment of an exploited coastal pike (Esox lucius) stock (Rügen, southern Baltic Sea) underscores the crucial relevance of growth and natural mortality for assessment outcomes

Many capture fisheries are considered data-poor, including the northern pike (Esox lucius) fishery in brackish lagoons in the southern Baltic Sea of Germany. The objective of our work was to assess the exploitation status of this stock, which is perceived by stakeholders to be in decline. Size structure data collected via rod-and-reel-angling, fyke nets, and gill nets, and empirical estimates of growth, maturation, and fecundity from the lagoon stock were used to fit the Length-Based Spawning Potential Ratio (LB-SPR) model. Parameter uncertainty in von Bertalanffy growth estimates and natural mortality in the Baltic Sea pike stock was considered in sensitivity analyses. Assessment outcomes were sensitive to estimates of growth rate, particularly asymptotic length L∞, instantaneous natural mortality M, and gear selectivity. Under-aging of old fish in scale-based age estimates overestimated terminal length and generated negative bias in the estimated stock status. Despite the sensitivity of assessment outcomes to life-history parameter choice, the stock status for the Baltic Sea consistently indicated a fully exploited situation with SPRs robustly above 0.4 and current fishing mortality rates between 0.2 and 0.4 yr−1. This result agreed with previous assessments using catch-only models. Our work serves as a reminder, that when using length-based methods, unbiased growth, and natural mortality estimates are critical for robust assessment outcomes.

Assumptions influencing the estimation of natural mortality in a tag-integrated statistical model for western and central Pacific Ocean skipjack

The status of skipjack tuna (Katsuwonus pelamis) in the western and central Pacific Ocean is regularly assessed using the integrated catch-at-age model MULTIFAN-CL. The model utilizes catch, fishery-dependent standardized CPUE indices, unstandardized effort, mark-recapture, and length-composition data. We tested the sensitivity of natural mortality estimation to functional forms of M at age, assumptions of tag reporting rate parameters, and modeling decisions that influence the fit to the length composition and tag data. We estimated M at age internal to the tag-integrated spatially-explicit model using a range of parameterizations. Likelihood component profiles for mean natural mortality values, nodes in a cubic spline of M at age, and reporting rate parameters were conducted to determine which data were contributing to the estimation of these parameters. The functional form of M at age did not have a large influence on performance indicators (Frecent/FMSY and SBrecent/SBF=0), but cubic splines proved a parameter efficient method of estimation. Assumed functional forms of selectivity and the fixed growth curve influenced estimates of M at age and had moderate impacts on performance indicators, though the weighting of the length composition data was less influential. Some reporting rate parameters were estimated on the upper bound for all models; changes to assumptions of these parameters impacted performance indicators slightly more than length composition changes, but did not strongly influence M-at-age estimates. Assumptions that affected the amount of tagging data included in the model, such as the mixing period, had a large impact on the performance indicators and shape of M at age, but only moderate changes to mean M estimates. Likelihood profiling showed that both tagging data and length composition data strongly influenced estimates of M at age in this catch-at-length tag-integrated model, but there is conflict among the data sources and tagging programs. Standard errors of M at age were small for most ages, particularly those with numerous tag recaptures, and mean M estimates across tested sensitivity models were mostly consistent. There is some concern that estimates may be biased due to incomplete mixing of tags and other assumption violations, but simulation studies are needed to determine the severity. Research is also recommended on methods to resolve data conflict that may be causing reporting rates estimated on upper bounds and to determine appropriate mixing periods of tagging data.

Stomach content characterisation of the marine range-shifting Octopus tetricus using DNA metabarcoding

The common Sydney octopus Octopus tetricus has undergone range extension along the east coast of Australia, associated with regional warming and the strengthening and southward extension of the East Australian Current (EAC). Its historical range of distribution is from southern Queensland to southern New South Wales, but it is now also found off north-east Tasmania, where it may affect local ecosystem dynamics due to changes in trophic interactions. This study aims to identify the prey and trophic level of O. tetricus from specimens collected off Tasmania to anticipate potential ecological, economic, and conservation effects in the range-extended area. The stomach contents of 18 O. tetricus individuals captured off north-east Tasmania in 2011 were characterised using DNA metabarcoding. Sixteen families of prey were identified; crustaceans (Alpheidae, Calcinidae, Diogenidae, and Galatheidae) were the most frequently detected prey, followed by fishes and bivalves. Prey species of commercial importance included blue-throated wrasse Notolabrus tetricus and the scallop Pecten fumatus. O. tetricus was found to have an intermediate trophic level of 3.66, participating in the transfer of energy from lower to upper trophic levels. Predation by O. tetricus could present competition to local fishers for resources and additional pressure on local fishery stocks, and may alter estimates of natural mortality used by fisheries management. The present study may be useful to scientists, fishery managers, and conservationists because it provides a preliminary assessment of the diet of O. tetricus, with potential ecological, economic, and conservation implications in the range-extended area.

Regional Variation in Growth and Mortality of Spotted Seatrout in the Western Gulf of Mexico

AbstractThe Spotted Seatrout Cynoscion nebulosus, a year‐round estuarine resident throughout the Gulf of Mexico, is a common mid‐trophic predator across its range and is one of the most popular game fish in Texas. In 1998, the Texas Parks and Wildlife Department initiated long‐term collections of otoliths from Spotted Seatrout encountered in fishery‐independent samples. Here, we use data from those collections (1998–2018) to generate a coastwide growth function and assess regional and temporal variation in growth and mortality. Key findings from this work include (1) sexual dimorphism in Texas that is consistent with previous studies across the species' range, with females exhibiting faster growth rates (growth rate parameter k) and having a greater asymptotic length than males based on coastwide sex‐specific growth functions; (2) relatively fast growth in northern estuaries, as evidenced by estuary‐specific patterns of deviation from a coastwide growth function as well as variability among k‐values from regionalized growth functions; and (3) higher estimates of natural mortality and total mortality in northern estuaries based on regional variation in k and catch‐curve analysis. The third finding was supported by an increase in estuary‐specific mean age of Spotted Seatrout from north to south, suggesting that older fish are more common in southern estuaries. Broadly, significant regional variation in age, growth, and mortality implies a complex underlying demographic structure. Clinal variation in important life history phenotypes of Spotted Seatrout in the western Gulf is consistent with previous studies that have described clinal genetic variation for the species, implying the potential for overlapping but independent subpopulations in the western Gulf of Mexico.

Implications of the maximum modelled age on the estimation of natural mortality when using a meta-analytic prior: The example of eastern Australian orange roughy (Hoplostethus atlanticus)

The eastern Australian stock of orange roughy (Hoplostethus atlanticus) is a deep-water, long-lived species with a history of considerable exploitation during the late 1980s and early 1990s before being reduced to around 10% of unfished spawning biomass only a few years later, resulting in the closure of the fishery in 2006. Recent assessments have shown an increase in biomass, and the fishery was re-opened to targeted fishing in 2015. The stock is an example of the consequences of over-exploitation and of managed recovery. As such, conservation groups and fisheries managers are very interested in the status and future of the stock. Consequently, the assessment is both contentious and highly scrutinised. The current assessment uses the Stock Synthesis platform, with key inputs being annual catches, occasional acoustic surveys and age-composition data. Natural mortality (M) has been fixed at 0.04 yr−1 in the model on which management advice was based for several assessment cycles, but the maximum likelihood estimate of M is closer to 0.03 yr−1. Reducing the assumed value for M would lead to large reductions in catch, as determined by the Australian harvest control rule. A prior for M was developed based on assessments of orange roughy stocks in New Zealand and included in a Bayesian analysis in which M was treated as an estimable parameter. The median of the posterior for M is 0.0353 yr−1 when the maximum age in the assessment (i.e., the ‘plus-group age’) is set to 80 years, but setting the plus-group age to 80 is not based on analysis of data. Increasing the plus-group age to 100 and 120 years leads to posterior medians for M of 0.0381 and 0.0393 yr−1 respectively. This has consequences for catch limits under the Australian harvest control rule, with models that have older plus-group ages having higher estimated productivity and recommended biological catches. The results highlight the value of making use of the results from assessments of similar species to develop priors for M, especially for species that are poorly represented in the data sets on which current meta-analyses are based, and for the need to consider the choice of plus-group age when conducting assessments, particularly for long-lived species.

Approaches for estimating natural mortality in tuna stock assessments: Application to global yellowfin tuna stocks

The values used for natural mortality (M) are very influential in stock assessment models, affecting model outcomes and management advice. Natural mortality is one of the most difficult demographic parameters to estimate, and there is often limited information about the true levels. Here, we summarise the evidence used to estimate natural mortality at age for the four main stocks of yellowfin tuna (Indian, Western and Central Pacific, Eastern Pacific, and Atlantic Oceans), including catch curves, tagging experiments, and maximum observed age. We identify important issues for estimating M such as variation with age linked to size, maturity state or senescence, and highlight information gaps. We describe the history of natural mortality values used in stock assessments by the tuna Regional Fisheries Management Organisations responsible for managing each stock and assess the evidence supporting these values. In June 2021, an online meeting was held by the Center for the Advancement of Population Assessment Methodology (CAPAM), to provide advice and guidance on practices for modelling natural mortality in fishery assessments. Based on approaches presented and discussed at the meeting, we develop a range of yellowfin tuna natural mortality estimates for each stock. We also recommend future research to improve these estimates of natural mortality.

Informing Management of Atlantic Bluefin Tuna Using Telemetry Data

Sustainable management of exploited marine fish and wildlife populations requires knowledge about their productivity. Survival from natural causes of mortality is a key component of population productivity, but is notoriously difficult to estimate. We evaluate the potential for acoustic telemetry as a long-term monitoring tool to estimate rates of natural mortality. We present a Bayesian multistate mark-recapture model for telemetry data collected over a decade from 188 Atlantic bluefin tuna (Thunnus thynnus) and apply it to estimate the rate of natural mortality using only acoustic tag detections for all animals, or using acoustic tag detections for 96 single-tagged tuna plus acoustic tag detections combined with estimated positions from pop-up satellite archival tags for 92 double-tagged animals. We support the model for bluefin tuna with a simulation study to quantify bias in estimates of population dynamics parameters and investigate the effect of auxiliary information from satellite tagging on mortality rate estimates for different acoustic tag detection probability scenarios.We obtained posterior estimates of the instantaneous annual natural mortality (survival) rate across a decade of tagging for Atlantic bluefin tuna of 0.17 yr-1 (0.84 yr-1) both using only acoustic tagging data, and using a combination of acoustic and satellite tagging data. Use of a prior implying a higher rate of fishing mortality yielded an instantaneous annual natural mortality (survival) estimate of 0.10 yr-1 (0.90 yr-1), with combined acoustic and satellite tag data. Results from the simulation study indicate that the use of satellite tags can improve the precision and accuracy of estimates of detection probabilities, area-specific movement probabilities and mortality rates, where the extent of the improvement depends on true underlying acoustic tag detection probabilities. Our work demonstrates that long-term acoustic tagging data sets have strong potential for monitoring of highly migratory marine fish and wildlife populations, providing information on a number of key parameters, including survival and movement rates. However, improved information on tag reporting rates or fishing mortality is needed to better separate natural and fisheries mortality for Atlantic bluefin tuna.

Upgrading from M version 0.2: An application-based method for practical estimation, evaluation and uncertainty characterization of natural mortality

Natural morality (M) is a notoriously difficult population parameter to estimate, yet it is also one of the most important measures of life history that sets, as Beverton and Holt called it, "the course of events". Stock assessments that include this parameter often show great sensitivity to its value, reflecting the need to characetrize the uncertainty inherent in its estimation. Direct measurement of natural mortality is often limited to resource intensive tag-recapture studies. Indirect measures are more often used, and are built on life history theory, relating natural mortality to traits such as age, size, maturity and reproductive condition (or just assuming 0.2). The Natural Mortality Tool attempts to accumulate several empirical estimators of M into one application. Users simply input life history values to obtain estimates of natural mortality. These estimates can be taken individually or can be combined into a weighted density function that can be used to develop an M prior that integrates uncertainty across several estimates. Comparing estimators can also reveal inconsistencies in life history values that may lead to further refining of basic biological understanding. Two examples are used to demonstrate tool functionality and highlight general recommendations on implementation. Making these estimators and the development of uncertainty in estimating natural mortality more widely available hopefully supports transparent and defensible decision-making on the treatment of this important population parameter.

Times are changing, but has natural mortality? Estimation of mortality rates for tropical tunas in the western and central Pacific Ocean

A variety of density-dependent and -independent processes have been proposed to influence natural mortality rates, potentially leading to variation through time. Processes of natural mortalities are rarely directly observed, making estimation of natural mortality rates difficult. Mark-recapture data allow estimation of total mortality rates, which can be separated into natural and fishing mortality with information on rates of tag reporting, tag shedding and tag-induced mortality. We fitted attrition models and length-based Brownie models to four decades of mark-recapture data from skipjack and yellowfin tuna in the Western and Central Pacific Ocean, a period representing a sustained expansion of associated fisheries in the region as well as rapid changes to the marine environment. The modelled dataset included c. 250,000 skipjack and 100,000 yellowfin tag releases, with 45,000 and 17,000 recoveries of skipjack and yellowfin respectively, released from 1977 to 2017. Increases in fishing mortality were detected over this time for both skipjack and yellowfin, with evidence of temporal changes in selectivity for yellowfin. Estimates of natural mortality were highest for the smallest size class and generally lower for larger sizes, though there was large uncertainty in the largest size groups due to lower sample sizes of tagged fish. There was no clear evidence of temporal changes in natural mortality rates for either species, though there was some evidence of changes in natural mortality for the smaller yellowfin size classes (< 61 cm). However, there was likely insufficient statistical power to test for plausible changes in natural mortality rates for yellowfin due to low precision of estimates during the earlier years of the tag dataset.

Estimating natural mortality for Atlantic Sea scallops (Placopecten magellenicus) using a size-based stock assessment model

We estimate temporally varying natural mortality within a size-based stock assessment model for U.S. sea scallops in three different regions, and compare the results to corresponding models where natural mortality is not estimated, and (in one case) where only the mean natural mortality is estimated. In one model, for the Georges Bank Closed Areas, natural mortality was estimated by year for all sizes, whereas in the other two models (for the Georges Bank Open Areas and the Mid-Atlantic Bight), annual variation in natural mortality was estimated for juveniles only. Estimating natural mortality by year improved the performance over fixed natural mortality in all three models, as measured by Akaike’s information criterion (AIC) and reduced retrospective patterns. There was evidence that natural mortality is density-dependent. Large recruitment events were associated with substantial increases in juvenile natural mortality, and natural mortality in the Georges Bank Closed Areas tended to increase at high biomass. Estimation of natural mortality was facilitated by survey data that gave estimates of absolute abundance together with low fishing mortality that minimized the confounding between natural and fishing mortality. Natural mortality is an important process that should be estimated when there is sufficient information to do so.

Evaluation of natural mortality of an unfished gastropod (Buccinum undatum) population using statolith age‐frequency data in the southern Mid‐Atlantic Bight

AbstractNatural mortality estimates of harvested stocks are key parameters used in stock assessments to aid in understanding the dynamics of populations, but these estimates are difficult to obtain and have a high degree of uncertainty. Data from unfished populations, which is often rare, provide a unique and valuable opportunity to estimate natural mortality. This study provides estimates of natural mortality using multiple methods from an unfished whelk (Buccinum undatum) population using statolith ages which prove to be an accurate method to assess age of gastropods. Using statolith age‐frequency data, natural mortality for this unexploited Mid‐Atlantic Bight whelk population was estimated to be 0.45–0.60 year−1. Due to the unexploited state of this population, the mortality estimate in this study can be assumed to be a true reflection of natural mortality and thus compared with mortality estimates for populations under varying degrees of exploitation to understand how exploitation affects population dynamics.

Simple length‐based approaches offer guidance for conservation and sustainability actions in two Central American small‐scale fisheries

Abstract Many species targeted by multi‐gear small‐scale fisheries in developing countries are poorly studied in terms of fisheries sustainability even as their contributions to biodiversity, livelihoods, export earnings and food security are well documented. This study presents new information on more than 150 fish species that constitute the bulk of small‐scale fishery landings in Belize and on the Pacific coast of Guatemala to provide guidance for improved biodiversity conservation and fisheries management. This includes a number of species listed as threatened on the IUCN Red List and Appendix II of CITES. Simple, length‐based approaches and thresholds of size at maturity show that many of the elasmobranch and bony fish species examined have many immature individuals landed (Belize 51%, Guatemala 77%) across several gears. In both countries, maximum size is negatively correlated with the intensity of overfishing for several common metrics: proportions of mature, optimal sized and mega‐spawners. The disproportionate removal of the largest species first will have cascading effects on biodiversity, community structure and species interactions, as well as livelihood opportunities and food security for fishing communities. Status indicators, such as fishing mortality rate relative to natural mortality rate and estimates of depletion based on lengths are sensitive to the assumed values of life history parameters, indicating the importance of improved data on growth, maturity and mortality for these populations. Our study emphasizes the use of a combination of fisheries conservation strategies, including size and effort controls, in conjunction with the closed area concept (i.e. marine protected areas) and meaningful consultation with resource users in order to maximize benefits for nature and people.

A systematic map of demographic data from elephant populations throughout Africa: implications for poaching and population analyses

Abstract The crisis facing Africa’s elephant populations is a notorious example of ongoing wildlife declines caused by illegal harvesting. Targeted conservation interventions require detailed knowledge about changes in population sizes and the effect of illegal activities. However, accurately quantifying poaching intensity is a difficult task: commonly calculated from ranger‐based carcass‐encounter data, the proportion of illegally killed elephants (PIKE) is a function of poaching and background mortality. Hence, at constant poaching intensity, PIKE decreases with increasing natural mortality and also with hunting, management interventions, and other anthropogenically induced deaths. Natural mortality is often more difficult to quantify with accuracy than mortality due to illegal killing, as elephants that die naturally are more likely to be missed than those taken by poachers. In recent analyses, constant background mortality rates were assumed. Yet, for example climate‐driven fluctuations in natural mortality, if not quantified and accounted for, may lead to biased estimates of poaching intensity. Varying background mortality rates can be accounted for in the analysis of PIKE, but this requires near‐complete counts of natural and management‐related deaths and hunting records. Carefully developed population models, which simulate population dynamics and demographic changes while accounting for variation in environmental conditions and management strategies, are alternatives. However, successful calibration of such models requires integrating comprehensive demographic data. We systematically review the scientific and ‘grey’ literature on African elephant demography with the objective of facilitating poaching and population analysis possibilities through an inventory of information relevant to demography. Our screening of 10900 publications resulted in the review of relevant information provided by 431 studies from 420 study sites throughout Africa. From these, we extracted demographic data collected between 1900 and 2017, and collated them in the newly created African Elephant Demographic Database (AEDD; 10.6084/m9.figshare.19387085). We found 37 natural mortality estimates from five different study sites. Other mortality data, demographic rates, and age‐ and sex‐structured population data were substantially more abundant, both temporally and spatially. This new collection of demographic rates, age‐ and sex‐structured population data, and cause‐partitioned mortality estimates identifies spatial and temporal data gaps and provides prior information needed for African elephant population models. Closing these data gaps and subsequent analyses of realistic population models may aid elephant conservation via improved policies, legislation, and protection.

Length-based approaches to estimating natural mortality using tagging and fisheries data: The example of the eastern Aleutian Islands, Alaska golden king crab (Lithodes aequispinus)

We evaluated natural mortality (M yr−1) estimation reliability for eastern Aleutian Islands male golden king crab (Lithodes aequispinus), an exploited stock, by comparing an integrated length-based assessment model approach and a likelihood method using tag release-recapture data. We used 1997, 2003, and 2006 tag release-recapture data to estimate the size transition matrix, M, fishing mortality, total selectivity, and retained selectivity and compared M likelihood profiles between both methods. Furthermore, we conducted simulations with varying initial survival and tag reporting rates to show that M estimation reliability declines as these rates decline. Restricted time-at-large, full initial survival, and complete reporting assumptions produce an M estimate closer to that of the integrated length-based assessment method. Because the reliability of the M estimate is sensitive to changes in these assumptions, future efforts should focus on maximizing the number of tagged crab (and hence recaptures), evaluating initial tag-induced mortality, and optimizing tagged crab recapture reporting rates. The reliance of fishery-dependent data and high tag reporting rates highlights the importance for maintaining a positive cooperation with fishers for efforts towards sustainable management.

Age, growth, and mortality of littlehead porgy, Calamus proridens, from southeast Florida coastal waters

Sectioned otoliths were used to age littlehead porgy Calamus proridens ( n = 737) from southeastern Florida recreational fisheries. Otolith sections contained easily identifiable annuli and were reliable structures for aging littlehead porgy up to 11 yrs. Opaque zones were deposited from March to June (peaking in April). Littlehead porgy ranged from 1 to 11 yrs and 220 to 385 mm total length (TL). Body size relationships were TL = 1.09 FL + 16.06 ( n = 3474, r2 = 0.95), FL = 0.87 TL – 1.15 ( n = 3474, r2 = 0.95), W = 4.76 × 10 –5 TL 2.80 ( n = 6722, r2 = 0.86), and W = 1.2 × 10 –4 FL 2.71 ( n = 3474, r2 = 0.90), where W is total weight (g) and FL is fork length (mm). Growth was not significantly different between two discrete geographic regions of Florida, and the von Bertalanffy growth equation was Lt = 335 (1 – e –0.41 ( t + 2.13) ) for all areas combined. The point estimate of natural mortality was M = 0.38, while age-specific estimates of M ranged from 0.60 to 0.41 yrs–1 for ages 1–11. Littlehead porgy were fully recruited to the headboat fishery by age- 3 and catch curve analysis resulted in an estimate of total mortality Z = 1.08 for ages 3–7+. This study reports first estimates of life history parameters for littlehead porgy from Atlantic waters of the southeastern United States.

Geographical variation in age and growth of the endemic Australian sciaenid Atractoscion atelodus.

Growth rates and other age-related population characteristics are essential parameters underpinning management of a stock. This is the first study to estimate length-at-age of Atractoscion atelodus (family Sciaenidae) in New South Wales (NSW), despite the species being exploited since the 1940s. The aim of the current study was to quantify the age-based biological characteristics of A. atelodus and in particular: (a) validate the use of sagittal otoliths to quantify age; (b) estimate the growth rates and longevity of males and females; (c) examine geographical variation in age and growth; (d) document the age composition in commercial landings and (e) estimate mortality rates. A. atelodus is relatively fast growing, reaching 40 cm fork length (LF ) in the first year of life and living to at least 14 years. Growth was significantly different between sexes, with modelled asymptotic lengths of females (L∞ =84.6 cm LF ) greater than males (L∞ =69.0 cm LF ). Growth rates were also significantly different between northern and southern populations. Fish sampled from the southern region were younger and faster growing than those sampled in the northern region, supporting the counter-gradient growth theory and the influence of upwelling providing greater nutrient and food availability. The commercial fishery was predominantly based on young fish <3 years. with few (c. 5%) greater than 5 years. Fishing mortality estimates (F =0.42) were similar to natural mortality estimates (M =0.44). The age-based parameters estimated in the present study suggest that A. atelodus should be relatively resilient to fishing; nonetheless, the sizes and ages in landings are indicative of a stock heavily fished. The fishery is experiencing truncated age distributions and appears to be largely recruitment driven, increasing susceptibility to overexploitation. Determining geographical differences in growth rates of a population has important implications when considering impacts of anthropogenic drivers such as global warming and overexploitation and is important to determine in managing exploited fish populations.

Exceptional longevity in a lightly exploited, semi-anadromous clupeid (Perth herring Nematalosa vlaminghi) within a degraded estuarine environment.

Many anadromous (and semi-anadromous) fish species, which migrate from marine to freshwater ecosystems to spawn and to complete their life cycle, are currently threatened by habitat degradation in the upper parts of estuaries and rivers, where spawning and juvenile nursery areas occur. This situation pertains to Nematalosa vlaminghi, a semi-anadromous gizzard shad (Clupeidae: Dorosomatinae) endemic to south-western Australia. More information on the biology of N. vlaminghi is required for its effective management and conservation. This study estimated growth, longevity and natural mortality of N. vlaminghi. Ages were determined by counting validated annual growth increments in thin sections of sagittal otoliths. Fish were sampled in the Swan-Canning Estuary, which historically hosted the main commercial fishery for N. vlaminghi. Since the late 1990s, however, only very minor catches of this species have been taken from this estuary and none since 2007. Given the essentially unexploited state of the current population, the estimate of total mortality (Z, y-1 ) from the catch curve analysis in this study provides a direct estimate of natural mortality (M, y-1 ) for N. vlaminghi. Somatic growth during this study was substantially slower than that historically reported for N. vlaminghi. Various processes operating in this estuary since the 1970s may have contributed to slower growth, including increased hypoxia, higher primary productivity due to eutrophication and cessation of fishing for N. vlaminghi. The maximum observed age of 19.8 years for N. vlaminghi is the highest reported for any gizzard shad globally and one of the highest reported for any clupeid species. This exceptional longevity is likely part of a life-history strategy that allows N. vlaminghi, which exhibits substantial variation in annual recruitment success, to persist in the intermittently closed estuaries of south-western Australia where environmental factors, including low flow and hypoxia, can create unfavourable conditions for reproduction for extended periods.

The Use of a Length-Structured Multispecies Model Fitted Directly to Data in Near-Real Time as a Viable Tool for Advice

Fish communities are multispecies assemblages, so ideally multispecies models should be used directly for assessing this resource. However, progress in this direction has been slow, partly because these models are often more complex and take longer to fit, rendering them too slow to demonstrate near-real-time utility, and thus creating a perception that they are opaque to stakeholders. In this study we implemented a multispecies assessment for the Irish Sea, fitting a model of eight key stocks directly to catch and survey data. Two novel features of our approach allowed the multispecies model to be fitted within a few hours. Firstly, by using size-based theory and life-history invariants to specify many of the required properties of stocks, we reduced the number of general parameters that needed to be fitted directly to a more manageable 25. Secondly, by using state-of-the-art fitting methods, we found acceptable solutions quickly enough to provide assessments within the timescale of an advisory meeting. The outcomes were compared with the standard single species assessments of the same eight species. Model fits were generally good for either catch or at least one of the surveys, but not for all data sources at the same time, illustrating the challenges of fitting multiple stocks to different data sources simultaneously. Estimates of SSB and F agreed qualitatively with the assessments for most stocks with the exception of whiting. Estimates of natural mortality showed modest year to year variability, suggesting that single species assessments may be appropriate for short term tactical management. This method shows great promise for making multispecies assessments as a complement to existing assessments because of the rapid turnaround time and ability to respond at meetings to the requests of stakeholders. In addition, because these models avoid our current dependence on existing single species models they can be used to provide boundary conditions in natural mortality for standard assessment models and check for internal consistency in the assessment process. Furthermore, they are easily accessible because they are based upon freely available code.