Instantaneous Rate Of Fishing Mortality Research Articles

A synthesis of a coastal northern pike (Esox lucius) fishery and its social-ecological environment in the southern Baltic Sea: Implications for the management of mixed commercial-recreational fisheries

We synthesize a large body of literature involving peer-reviewed work, grey literature and novel data analyses about the small-scale northern pike (Esox lucius) fishery in lagoon ecosystems in the southern Baltic Sea. Based on our comprehensive review that synthesizes ecological as well as social, economic and governance-related literature we derive implications for the management of mixed commercial-recreational fisheries in coastal areas. The interconnected shallow and biologically highly productive meso- to polytrophic lagoons (extension about 2000 km²) bordered by the peninsula of Fischland-Darß and the islands of Hiddensee, Rügen and Usedom in the southern Baltic Sea of Germany constitute an oligo- to mesohaline transitional habitat suitable for colonization by a range of freshwater fishes, including pike. In the Rügen area, pike successfully recruits in the mesohaline lagoons, but anadromous subpopulations and freshwater residents also exist in tributaries, forming a connected meta-population. The stock is co-exploited by a small-scale commercial fishery and a largely tourism-dominated recreational fishing sector that, depending on the angler type, values the pike for both consumption as well as for its trophy size. The recreational sector has risen in economic and social relevance since the German reunification in 1990 and today removes similar amounts of biomass than commercial fisheries. Pike is a prime target species of anglers, and recreational pike angling in the lagoons today generates a larger economic impact in terms of jobs created compared to the commercial pike fishing, where pike is typically one target among many freshwater fish. Stock assessments and stakeholder reports have revealed that the stock size and size of pike in the catch have been falling since 2010, fueling conflicts among fishers and anglers for space and fish. Reasons for the current decline of the pike stock involve multiple pressures operating jointly and possibly synergistically, such as local overharvest, loss of stock structure through past blocking of freshwater streams, eutrophication and macrophyte loss, predation mortality by natural predators, reduced availability of marine prey through declines of western Baltic spring-spawning herring (Clupea harengus), and poorly understood impacts of climate change. The assessment of current fishing mortality suggests a the stock stock is size/quality-overfished from the perspective of anglers, and fully exploited to slightly growth-overfished when judged against the reference point of maximum sustainable yield. The current biomass trend is negative. The current instantaneous fishing mortality rate, F, is estimated to range between 0.2 and 0.4 yr-1. Hence, fishing cannot be the sole culprit of the current stock decline because the current fishing mortality rates are too high for the underlying productivity, but not excessive. Because the current fishery no longer meets the expectations of recreational anglers, and angler numbers have recently declined, if the aim is to also suit anglers, reductions of fishing mortality would be useful in recovering the stock and fishing quality, coupled with restoration of access to flooded wetlands as spawning and nursery grounds, and control of other mortality sources of pike (e.g., cormorants). However, whether such actions indeed rebuild the fishery remains uncertain because of the potential for compensatory natural and fisheries mortality and other environmental changes affecting recruitment and abundance negatively. Policy makers may want to solve the allocation problem among commercial and recreational fisheries, install a robust monitoring system and a management framework that is inclusive of multiple perspectives and objectives and adaptive to novel productivity regimes and further structural changes in the mixed fishery. Further research on climate change impacts, food web changes, impacts of natural predators such as seals, cormorants or stickleback, and the behavioral and socio-economic aspects of commercial and recreational fisheries is warranted.

Age, Growth, and Mortality of Atlantic Tripletail in the North‐Central Gulf of Mexico

AbstractIn the southeastern USA and the Gulf of Mexico (GOM), Atlantic Tripletail Lobotes surinamensis are increasingly targeted by recreational anglers, indicating that stock status should be assessed. A critical need for such assessments is age‐specific data; however, previous studies have drawn conflicting conclusions regarding the most appropriate structure for aging. Moreover, growth parameters and mortality rates for GOM Atlantic Tripletail are unknown. Therefore, the goals of this study were to (1) evaluate sagittal otoliths and first dorsal spines as aging structures; (2) model combined and sex‐specific growth; and (3) estimate mortality rates for GOM Atlantic Tripletail. From 2012 to 2019, Atlantic Tripletail (N = 230, including a near‐record‐size specimen) were collected from the north‐central GOM via hook and line and were aged using otoliths and first dorsal spines. Total length ranged from 212 to 940 mm, and age ranged from 0.07 to 5.27 years. Otoliths produced higher percent agreement (95.0%) and lower average percent error (3.0%) between readers compared to spines (82.9% and 6.5%, respectively). The von Bertalanffy growth parameters differed slightly between the otolith‐based data (mean asymptotic length [L∞] = 762.42 mm, Brody growth rate coefficient [k] = 0.69 year−1, and hypothetical age at which length equals zero [t0] = −0.58 year) and spine‐based data (L∞ = 718.83 mm, k = 0.79 year−1, and t0 = −0.56 year). For both otolith‐ and spine‐based sex‐specific data, the best‐fitting version of the von Bertalanffy growth function permitted L∞ to vary by sex. Chapman–Robson estimates of instantaneous total mortality rate and total annual mortality rate were 1.15 and 68.66%, respectively. Based on empirical, life history‐based methods, the instantaneous natural mortality rate was estimated at 0.75–0.97 and the instantaneous fishing mortality rate was estimated at 0.18–0.45, suggesting low levels of exploitation. These growth parameters and mortality estimates will provide information for future stock assessments, thereby ensuring sustainability of the GOM stock of Atlantic Tripletail.

Aspects of Population Dynamics of Pagrus Caeruleostictus Collected From the Continental Shelf of Sierra Leone and the Implications for Management

The study evaluated aspects of population dynamics of Pagruscaeruleostictus and the implications for management in Sierra Leone, needed to support efficient management plans of its fishery. 12, 729 specimens of Pagruscaeruleostictus were collected off Sierra Leone from January-November, 2016 using randomized sampling techniques on-board demersal trawler. Analysis of population dynamics employed methods fitted in FiSAT II software using pooled length-frequencies. Estimated growth parameters were asymptotic length (L∞ = 34.48cm) and weight (W∞ = 4693.6g), growth rate (K = 0.29yr-1), growth performance index (ɸ = 2.54), growth exponent (b = 3.09), theoretical age (to = -0.8years) and life-span (tmax = 9.50years). The instantaneous fishing mortality rate (F = 0.40yr-1) climaxed the optimum fishing mortality rate (Fopt = 0.30yr-1) and slightly below the limiting fishing mortality (Flimit = 0.51yr-1) whereas natural mortality rate (M) gave 0.76yr-1. Besides, the estimated current exploitation rate (Ecurrent) exceeded the estimated sustainable level of exploitation (E50 = 0.39yr-1) but below the estimated maximum allowable exploitation point (Emax = 1.10yr-1). Further, the length (Lm50 = 23.00cm) and age (tm50 = 4.60years) at first maturity climaxed the length and age at first capture (Lc50 = 21.64cm, tc50 = 3.8years,), length and age at first recruitment (Lr = 13.00cm, tr = 2.4years). Results portrayed slow and isometric growth, median life-span and high survival of a continuously recruited spawning stock of Pagruscaeruleostictus. Also, the stock suffered growth overfishing, moderate fishing pressure and overexploition, implying the need for management to recalibrate measures for rational exploitation.
 Res. Agric., Livest. Fish.7(3): 517-531, December 2020

The unknown Baranov. Forty years of polemics over the formal theory of the life of fishes

Abstract The year 2018 marked the 100th anniversary of the publication of paper “On the question of the biological basis of fisheries” by F.I. Baranov considered a cornerstone paper of modern fishery science. Baranov formalized population dynamics by describing changes in population abundance using differential equations, introducing the concept of instantaneous fishing and natural mortality rates, and developing his catch equation, which is the foundation of most modern age-structured stock assessment models. Baranov was the first to show the effect of fishing on population structure based on theoretical grounds. At the time of its publication, Baranov’s paper did not receive much attention in Russia and was completely unknown to scientists in the West. The second publication (On the question of the dynamics of the fishing industry, 1925) received substantial criticism from many and sparked a furious debate between Baranov and his opponents that lasted for several decades. The history and content of those debates, expressed in multiple papers by Baranov, is still largely unknown. I describe the essence of arguments by Baranov and his opponents. The story of these scientific debates reveals how different philosophical concepts and dominant points of view evolved through time.

Güney-doğu Karadeniz’in Fatsa kıyılarında tirsi balığının, Alosa immaculata Bennett, 1835 populasyon parametreleri

The aim of this study is to estimate population parameters of pontic shad, Alosa immaculata Bennett, 1835 in the Fatsa coast of the south-eastern Black Sea. A total of 314 pontic shad specimens were collected from study area using artisanal fishing gears from March 2013 to February 2014. In the study, parameters of the von Bertalanffy growth equation were found as L∞=43.05 cm; k=0.430 per year and t0=-0.451 year. The growth performance index (Φ') was estimated as 2.90. The total mortality (Z), natural mortality (M), fishing mortality rates (F) were calculated as 1.33 year-1, 0.75 year-1 and 0.58 year-1, respectively. The annual instantaneous fishing mortality rate was greater than both the target (Fopt=0.375 year-1) and limit (Flimit=0.50 year-1) biological reference points. Similarly, the present level of exploitation rate (E=0.43) was higher than the exploitation ratio for maximum yield per recruit (Emax=0.375) suggesting that overexploitation occurred. These results showed that this species has been over-exploited in the Fatsa coast of the south-eastern Black Sea. Measures should be taken to reduce the current exploitation rate for sustainable fishing of pontic shad in the Fatsa coast of the south-eastern Black Sea.

Growth, Mortality and Stock Assessment of Indian Mackerel, Rastrelliger kanagurta (Cuvier, 1816) in the Coastal Waters of Pakistan, Northern Arabian Sea

Population dynamics parameters of Rastrelliger kanagurta (Cuvier, 1816) were analyzed from January to December 2008 in coastal waters of Pakistan. FiSAT software was used to assess growth population dynamics parameters. The growth parameters K = 0.84 year–1 and L∞ = 24.57 cm were estimated with the aid of von Bertalanffy growth function. The natural mortality rate (M), the fishing mortality rate (F), and the total mortality rate (Z) were calculated as 1.682, 2.208, and 3.890 year–1, respectively. The annual instantaneous fishing mortality rate in our study was considerably greater than both the target (Fopt = 0.841 year−1) and limit (Flimit = 1.121 year−1) biological reference points, suggesting that this species is being over-exploited in the waters of Pakistan. An exploitation rate (E) was estimated to be 0.567 year–1and longevity (tmax) was calculated as 3/K equal to 3.57 years. The fishery of R. kanagurta by the gillnet catches in our study are of ages 2 to 3 years which indicated a reasonable mesh size for the catch.

Satellite telemetry reveals higher fishing mortality rates than previously estimated, suggesting overfishing of an apex marine predator.

Overfishing is a primary cause of population declines for many shark species of conservation concern. However, means of obtaining information on fishery interactions and mortality, necessary for the development of successful conservation strategies, are often fisheries-dependent and of questionable quality for many species of commercially exploited pelagic sharks. We used satellite telemetry as a fisheries-independent tool to document fisheries interactions, and quantify fishing mortality of the highly migratory shortfin mako shark (Isurus oxyrinchus) in the western North Atlantic Ocean. Forty satellite-tagged shortfin mako sharks tracked over 3 years entered the Exclusive Economic Zones of 19 countries and were harvested in fisheries of five countries, with 30% of tagged sharks harvested. Our tagging-derived estimates of instantaneous fishing mortality rates (F = 0.19-0.56) were 10-fold higher than previous estimates from fisheries-dependent data (approx. 0.015-0.024), suggesting data used in stock assessments may considerably underestimate fishing mortality. Additionally, our estimates of F were greater than those associated with maximum sustainable yield, suggesting a state of overfishing. This information has direct application to evaluations of stock status and for effective management of populations, and thus satellite tagging studies have potential to provide more accurate estimates of fishing mortality and survival than traditional fisheries-dependent methodology.

Statistical Catch‐Free Age‐Structured Assessment to Estimate Year‐Class Strength for Inland Recreational Fisheries: A Largemouth Bass Example

AbstractStatistical catch‐at‐age analysis (SCAA) is commonly used to estimate recruitment and mortality for large‐scale, economically important fish stocks, but fishery‐dependent catch‐at‐age data are often unavailable for smaller‐scale inland fisheries, thus limiting the use of SCAA. In these systems, a catch‐free approach to age‐structure modeling may be useful for estimating recruitment and mortality rates solely from a time series of survey age composition and catch rate data by estimating recruitment as a relative index of year‐class strength (Rt). Here, we demonstrate a statistical catch‐free age‐structured assessment (SCFASA) model for small‐scale inland fish stocks as an alternative to traditional catch curve approaches by using Largemouth Bass Micropterus salmoides as an example. We fitted the SCFASA to fishery‐independent survey age composition and catch‐per‐effort data from Largemouth Bass stocks in three Alabama reservoirs to estimate Rt, annual fishing mortality rates, and age‐specific electrofishing survey vulnerability; we compared Rt to catch curve residual estimates and performed a sensitivity analysis under a variety of model assumptions. Model estimates of Rt and temporal trends in fishing mortality were robust to changes in assumptions about the effective sample size of multinomial age samples, natural mortality rate, and temporal variability in fishing mortality. However, the magnitude of instantaneous fishing mortality rates at two of the three reservoirs was somewhat higher than empirical estimates for similar stocks and was sensitive to model assumptions. Our application of the SCFASA to survey data sets routinely collected by management agencies for Largemouth Bass potentially provides a comprehensive approach to estimating Rt, mortality, and sampling vulnerability for these stocks without increasing sampling effort. Future investigations of SCFASA's utility for small‐scale inland stocks should address the potential for bias in the magnitude of fishing mortality estimates due to incorrectly specified vulnerability schedules.Received October 5, 2015; accepted September 19, 2016 Published online December 21, 2016

Effects of changes in density-dependent growth and recruitment on sustainable harvest of lake whitefish

Abstract Substantial declines in growth and recruitment of lake whitefish ( Coregonus clupeaformis ) and changes in key density-dependent relationships since the 1990s have raised concerns about the effects of these changes on valuable commercial fisheries in Lake Huron. There is evidence for lake whitefish in the southern main basin of Lake Huron that growth and recruitment rates have been reduced by up to 50%. Using a life history model parameterized from fishery-independent survey data for lake whitefish, we investigated the effects of declines in growth and recruitment rates on population dynamics and sustainable harvest. We evaluated a baseline scenario characterized by high growth and recruitment rates, an alternative scenario with a reduced growth rate, and another alternative scenario in which both growth and recruitment rates were reduced. Yield consistently declined by at least 71% in both alternative scenarios compared to the baseline scenario. Also, fishing became unsustainable when both growth and recruitment rates were reduced and the maximum instantaneous fishing mortality rate exceeded 0.5. Our results suggest that the recent reductions in growth and recruitment observed in Lake Huron are of sufficient magnitude to alter productivity and reduce how much can be sustainability harvested from these stocks.

Estimating walleye (Sander vitreus) movement and fishing mortality using state-space models: implications for management of spatially structured populations

Fish often exhibit complex movement patterns, and quantification of these patterns is critical for understanding many facets of fisheries ecology and management. In this study, we estimated movement and fishing mortality rates for exploited walleye (Sander vitreus) populations in a lake-chain system in northern Michigan. We developed a state-space model to estimate lake-specific movement and fishery parameters and fit models to observed angler tag return data using Bayesian estimation and inference procedures. Informative prior distributions for lake-specific spawning-site fidelity, fishing mortality, and system-wide tag reporting rates were developed using auxiliary data to aid model-fitting. Our results indicated that postspawn movement among lakes was asymmetrical and ranged from approximately 1% to 42% per year, with the largest outmigration occurring from the Black River, which was primarily used by adult fish during the spawning season. Instantaneous fishing mortality rates differed among lakes and ranged from 0.16 to 0.27, with the highest rate coming from one of the smaller and uppermost lakes in the system. The approach developed provides a flexible framework that incorporates seasonal behavioral ecology (i.e., spawning-site fidelity) in estimation of movement for a mobile fish species that will ultimately provide information to aid research and management for spatially structured fish populations.

Effectiveness of a Refuge for Lake Trout in Western Lake Superior II: Simulation of Future Performance

AbstractHistorically, Lake Superior supported one of the largest and most diverse Lake Trout Salvelinus namaycush fisheries in the Laurentian Great Lakes, but Lake Trout stocks collapsed due to excessive fishery exploitation and predation by Sea Lampreys Petromyzon marinus. Lake Trout stocking, Sea Lamprey control, and fishery regulations, including a refuge encompassing Gull Island Shoal (Apostle Islands region), were used to enable recovery of Lake Trout stocks that used this historically important spawning shoal. Our objective was to determine whether future sustainability of Lake Trout stocks will depend on the presence of the Gull Island Shoal Refuge. We constructed a stochastic age‐structured simulation model to assess the effect of maintaining the refuge as a harvest management tool versus removing the refuge. In general, median abundances of age‐4, age‐4 and older (age‐4+), and age‐8+ fish collapsed at lower instantaneous fishing mortality rates (F) when the refuge was removed than when the refuge was maintained. With the refuge in place, the F that resulted in collapse depended on the rate of movement into and out of the refuge. Too many fish stayed in the refuge when movement was low (0–2%), and too many fish became vulnerable to fishing when movement was high (≥22%); thus, the refuge was more effective at intermediate rates of movement (10–11%). With the refuge in place, extinction did not occur at any simulated level of F, whereas refuge removal led to extinction at all combinations of commercial F and recreational F. Our results indicate that the Lake Trout population would be sustained by the refuge at all simulated F‐values, whereas removal of the refuge would risk population collapse at much lower F (0.700–0.744). Therefore, the Gull Island Shoal Refuge is needed to sustain the Lake Trout population in eastern Wisconsin waters of Lake Superior.Received November 14, 2014; accepted July 7, 2015

Population dynamics parameters of narrow-barred Spanish mackerel, Scomberomorus commerson (Lacèpéde, 1800), from commercial catch in the northern Persian Gulf

Population dynamics parameters of Scomberomorus commerson in Bushehr area waters of Iran were analyzed between October 2011 and September 2012. Fork length frequencies were collected from the gill net commercial catch. Von Bertalanffy growth function was used to estimate growth parameters K and L∞. Instantaneous total mortality rate (Z), instantaneous natural mortality rate (M) and the instantaneous fishing mortality rate (F) were also calculated. Resource status was evaluated by comparing estimates of the fishing mortality rate with target (Fopt) and limit (Flimit) biological reference points. FiSAT program was used to assess growth and mortality parameters. Based on the growth curve analysis, growth parameters were: K=0.5year−1 and L∞=148cm. Instantaneous total mortality was Z=0.97year−1. The estimate of M=0.56year−1 and F=0.41year−1, resulting in an exploitation rate (F/Z) of E=0.42year−1. Target and limit biological reference points were: Fopt=0.28year−1 and Flimit=0.37year−1. Size at capture at probabilities of 0.25 (L25), 0.5 (L0.5) and 0.75 (L0.75), were 46.3 cm, 55 cm and 60.2 cm respectively. Longevity (Tmax) was calculated as 3/K equal to 6 years. The exploitation rate (E) did not indicate overfishing; however, Fopt and Flimit rates were lower than fishing mortality (F), suggesting that overexploitation occurred. Growth parameters in the present study showed that the gillnet fishery catches of S. commerson that are of ages 1 or 2, which may not have reached maturity. Gillnet mesh size can be a factor why the catch is composed of mostly small, immature fish. Therefore, changes in mesh size and net size should be considered.

A size- and age-structured model to estimate fish recruitment, growth, mortality, and gear selectivity

Most stock assessment models are solely age-based and ignore variability in length-at-age, but the incorporation of length structure can improve parameter estimates. Here, we present an age- and length-structured population model to estimate fish recruitment, von Bertalanffy growth parameters, natural mortality, and gear selectivity from survey length- and age-specific catch, fishery length distributions, and total harvested biomass. We evaluated parameter bias when fitting the model to simulated data with variable fish longevity, survey sampling duration (i.e., years of data), survey gear selectivity (i.e., asymptotic or dome-shaped), instantaneous fishing mortality rate and fishing mortality trend (i.e., stable or increasing). Median proportional bias in mortality (fishing and natural), growth, and gear selectivity parameters did not exceed 0.05 across all scenarios. Median recruitment bias ranged up to 0.12 and was greatest for the short-lived species and for scenarios with increasing fishing mortality, but bias for all other scenarios was less than 0.1. The model was demonstrated using data from a whole-lake biomanipulation of gizzard shad ( Dorosoma cepedianum) at a 2300-ha lake in Florida, USA. Model estimates of gizzard shad exploitation rate were similar to an independent estimate from an in-season depletion analysis. Estimates of gizzard shad natural mortality and growth parameters were within the range of literature values for the species. The model could be useful for estimating growth, mortality, and recruitment parameters that account for gear selectivity and the cumulative effects of size-selective harvest.

Estimates of fishing and natural mortality rates for four Lake Whitefish stocks in Northern Lakes Huron and Michigan

We analyzed tag-recovery data to estimate instantaneous fishing ( F) and natural mortality ( M) rates of four lake whitefish stocks in lakes Michigan and Huron during 2004–2007. We tagged and released 22,452 adult lake whitefish of which 8.7% were subsequently recovered. Annual tag-reporting rates ranged from 17.8% to 56.2%. Tag retention was high for the first 5–6 months after tagging, but tag loss increased substantially thereafter. Nine tag-recovery models were evaluated with respect to whether F and/or M varied among stocks, lakes, or years. There was support for three models based on Akaike information criteria. The best model had yearly and stock-specific estimates of F of 0.03 to 0.79 and lake-specific estimates of M of 0.35 for Lake Michigan and 0.60 for Lake Huron. The second best model had yearly and stock-specific estimates of F of 0.04 to 0.71 and a constant estimate for M of 0.52. The third model had yearly and stock-specific estimates of F of 0.04 to 0.85 and stock-specific estimates of M of 0.32 to 0.67. Model-averaged estimates of F ranged from 0.04 to 0.78 and were substantially different than statistical catch-at-age estimates of F. Model-averaged estimates of M ranged from 0.40 to 0.59 and were greater than estimates obtained from prediction equations, possibly due to sea lamprey-induced mortality. We recommend that tag-recovery estimates of F and M be used as Bayesian priors in future lake whitefish stock assessments to help refine mortality estimates for the stocks.

Distribution and ontogenetic shifts in habitat and abundance of the temperate western blue groper, Achoerodus gouldii (Richardson)

Densities of juveniles western blue groper Achoerodus gouldii on exposed coasts of South Australia were slightly greater on granite than other substrata and increased with increasing bottom relief and declined with depth and increasing exposure. Sub‐adults were in higher density on schists than on other substrata, and densities increased with increasing bottom relief, and declined with depth, increasing exposure and increasing algal canopy cover. Adults were in higher densities on schists, and abundances increased with depth and bottom relief. At a mesoscale A. gouldii was absent from gulfs and sheltered bays, except near their entrances, while at offshore islands juveniles were rare, sub‐adults were in lower abundance, and adults in greater abundance, compared with mainland coasts. It is hypothesized that sub‐adults may swim up to 60 km from settlement locations to exposed areas and to offshore islands. At a geographic scale, abundances of all three size classes of A. gouldii declined significantly from west to east over 2000 km of coast from the centre of the species’ range to its eastern edge. Although A. gouldii is a protected species over part of its range, abundance of sub‐adults and adults was significantly reduced by fishing, as shown by an index of fishing intensity. The mean size of sub‐adult A. gouldii was also correlated with this index, suggesting that the observed geographic trend in sub‐adult and adult abundance may be contributed to by fishing. Simulation showed that a high instantaneous fishing mortality rate of 0·8 per annum could explain the maximum observed reductions in mean size of sub‐adult A. gouldii.

Biology and Population Dynamics of the Goby Pseudapocryptes elongatus in the Coastal Mud Flat Areas of the Mekong Delta, Vietnam

This study describe the reproductive biological characteristics and population parameters of the goby, Pseudapocryptes elongatus (Cuvier, 1816), in the coastal mud flat areas of the Mekong Delta, Vietnam. A total of 1058 specimens was collected from January 2004 to June 2005 and results showed that the breeding season occurred with two spawning peaks in July and October. Length at first maturity (L(m)) was 15.4 and 16.3 cm for females and males, respectively. The batch fecundity estimates ranged from 2,652 to 29,406 hydrated oocytes per ovary in the fish ranging from 12.8 to 22.4 cm TL. Length frequency data of the goby ranging from 9.0 to 24.0 cm TL were analyzed using the FiSAT II software. The von Bertalanffy growth parameters were determined as L8 = 25.9 cm, K = 0.66 year(-1) and t(o) = - 0.26 year(-1). The longevity (t(max)) of the goby was estimated to be 4.55 years. There were two recruitment peaks with very different magnitudes and the means of these two peaks were separated by an interval of 5 months. Length at first capture (L(c)) was 10.05 cm, the instantaneous fishing mortality rate (F = 1.38 year(-1)) and natural mortality rate (M = 1.46 year(-1)) accounted for 49 and 51% of the total mortality (Z = 2.84 year(-1)), respectively. Relative yield-per-recruit and biomass-per-recruit analyses gave E(max) = 0.65, E0.1 = 0.55 and E0.5 = 0.33. Results show that the fish stock is subjected to growth overexploitation.

Seasonal Natural and Fishing Mortality of Striped Bass in a Southeastern Reservoir

AbstractReliable estimates of natural and fishing mortality are important for management of exploited fish populations, but these components of the total mortality rate can be difficult to determine by traditional fisheries methods. We used telemetry data to determine seasonal instantaneous fishing (F) and natural mortality (M) rates of subadult and adult striped bass Morone saxatilis in Badin Lake, North Carolina. Our analyses were based on the fates of 64 fish implanted with sonic transmitters and released and tracked for 2 years. Natural mortality was low and constant during the course of the study (M ± SE = 0.10 ± 0.01) and was similar to estimates for other reservoir populations of striped bass. A natural mortality rate of 0.09–0.16 may be a reasonable approximation for populations across the southeastern United States. Fishing mortality varied seasonally and was highest in the spring and summer of 2002 and the summer and fall of 2003; annual fishing mortality (F ± SE) was 0.65 ± 0.08 in 2002 and 0.77 ± 0.08 in 2003. Due to these high harvest rates, estimated annual survival rates were low for the Badin Lake population (47% in 2002; 42% in 2003). Results of a yield‐per‐recruit model suggest that harvest of older, larger individuals can be increased in Badin Lake with a decrease in fishing mortality or a moderate increase in the minimum size limit, even when the effect of catch‐and‐release mortality of fish below the size limit is considered. Our results also indicate important considerations for researchers using this method in the future, including the need to estimate downstream emigration and delay the inclusion of newly tagged fish in analysis to avoid biasing estimates of fishing mortality.

Preliminary assessment of the biology and fishery for the narrow-barred Spanish mackerel, Scomberomorus commerson (Lacépède, 1800), in the southern Arabian Gulf

The population biology and fishery for Scomberomorus commerson in the southern Arabian Gulf were investigated using a combination of size frequency, biological and size-at-age data. Transverse sections of sagittal otoliths showed structural increments consisting of alternating translucent and opaque bands, which were used to estimate age. Edge analysis revealed an annual periodicity of formation with opaque zones being deposited between May and July in association with increasing seawater temperatures. The maximum absolute age estimates were 16.2 years (males) and 15.3 years (females). Initial growth was rapid with fish reaching more than half the asymptotic size by their second year and there were no significant differences in growth characteristics between sexes. Parameter values of the von Bertalanffy growth function fit to size-at-age data (males and females combined) were: k = 0.21, L ∞ = 138.6 cm ( L F) and t 0 = −1.9 years. Spawning occurred between April and August, the mean sizes and ages at first sexual maturity were 72.8 cm L F (1.9 years) for males and 86.3 cm L F (2.1 years) for females. The size at which fish were fully recruited to the fishery (62.6 cm L F) was considerably smaller than both the mean size at first sexual maturity for females and the size at which yield per recruit would be maximised (95.6 cm L F). Furthermore, the annual instantaneous fishing mortality rate of 0.62 year −1 (0.46–0.79 year −1 95% CI) was by far in excess of the precautionary target ( F opt = 0.13 year −1) and limit ( F limit = 0.17 year −1) biological reference points, indicating that the resource is heavily over-exploited. The results suggest that an increase in mesh size regulations for gillnets in combination with a substantial reduction in fishing effort will be required if resource conservation and stock rebuilding objectives are to be achieved. The study also highlights the need for a strategic regional approach to the assessment and management of this highly migratory species.

Eggs-per-recruit model for management of the California market squid (Loligo opalescens) fishery

We develop a per-recruit model for the management of the California market squid (Loligo opalescens) fishery. Based on recent confirmation of determinate fecundity in this species, we describe how catch fecundity (i.e., eggs remaining in the reproductive tracts of harvested females) can be used to simultaneously infer fishing mortality rate along with management reference points such as yield-per-recruit, spawned eggs-per-recruit, and proportional egg escapement. Rates of mortality and egg laying have important effects on these reference points. Somewhat surprisingly, increasing the rate of natural mortality decreased spawned eggs-per-recruit while increasing proportional egg escapement. Increasing the rate of egg laying increased both spawned eggs-per-recruit and egg escapement. Other parameters, such as the maturation rate and gear vulnerability of immature females, affected the reference points. In actual practice, the influence of these parameters for immature squid may go undetected if immature squid are excluded from analysis of the catch. Application of this model to routine management is feasible but requires refinement of sampling procedures, biological assumptions, and model parameters. This model is useful because it is grounded on empirical data collected relatively inexpensively from catch samples (catch fecundity) while allowing for the simultaneous calculation of instantaneous fishing mortality rate and egg escapement.

Fishing and Natural Mortality of Adult Largemouth Bass in a Tropical Reservoir

AbstractLargemouth bass Micropterus salmoides, an intensively studied sport fish of temperate North America, has been introduced into tropical regions, but with little evaluation of fishing or natural mortality rates. Previous research in Puerto Rico suggested that annual mortality rates are high, but whether the extended spawning season (nearly 6 months annually), high angler harvest, or a combination was the cause of the elevated mortality was unresolved. Using ultrasonic telemetry, we tracked 44 adult largemouth bass over an 18‐month period to quantify patterns of natural, fishing, and total mortality. Using a general capture–recapture model, we estimated the annual instantaneous rates of fishing (0.584, SE = 0.164) and natural mortality (0.310, SE = 0.122). Natural mortality varied seasonally and generally increased during periods of spawning activity; however, fishing mortality was fairly consistent throughout the study. Given the relatively high fishing mortality rate, restrictions on harvest could probably be used to alter the age and size distribution of this population.