First-time Spawners Research Articles

Maternal spawning experience and thermal effects on offspring viability of chub mackerel and their influence on reproductive success

Maternal effects on offspring viability significantly contribute to the productivity and resilience of exploited marine fish populations. The temporal trend of biomass of chub mackerel (Scomber japonicus) in the western North Pacific showed cyclic fluctuation patterns, but it has been rapidly increasing since 2011. Although there are several assumptions regarding the mechanisms of the population dynamics of this species in these waters, there is a contradiction in the effects of thermal environmental conditions on recruitment success. We explored the effects of maternal age and/or previous spawning experience on egg and offspring traits of captive chub mackerel in response to thermal conditions. Moreover, from the relationship between the growth rate and temperature experienced by captive fish, the experienced temperatures of wild specimens were assessed. The sizes of eggs spawned from females aged 1–3 years were negatively correlated with temperature, but the effects of maternal age on egg size and nutrition were significant with larger and nutrient-rich eggs spawned from 2- or 3-year-old females (repeat spawners) than those from 1-year-old females (first-time spawners) at a given temperature. Such temperature- and maternal-age-dependent egg traits reflected the standard length of first-feeding larvae. The effects of maternal age on the starvation tolerance of larvae after hatching and body sizes of first-feeding larvae were also evident in larvae derived from 3-year-old females showing longer starvation tolerance and larger body heights than those from 1-year-old females. Further, larvae derived from 3-year-old females grew significantly faster than those from 1-year-old females at the tested temperatures. The process of ontogenetic development and growth rate of captive specimens until age 25 days were positively correlated with temperature. Meanwhile, the estimated temperatures experienced by wild specimens until age 25 days ranged from 18.0 to 20.3°C. These findings suggested that there were opposing thermal responses between maternal and offspring traits of chub mackerel, but the temperatures experienced by the main recruit fish may be restricted within the range of temperatures available for spawning. We discuss the mechanisms of reproductive success of this species in accordance with the maternal spawning experience and thermal effects on life history traits.

Evaluating growth dimorphism, maturation, and skip spawning of Atlantic halibut in the Gulf of Maine using a collaborative research approach

The data-limited nature of Atlantic halibut (Hippoglossus hippoglossus) in U.S. waters hampers evaluation of what may be a slow but steady rebuilding pattern. Here, we collaborate with the commercial fishery to design and implement a multi-gear sampling program that collected 100s of biological samples from throughout the Gulf of Maine in a five-year period, 2014–2018. Examination of sectioned otoliths revealed a maximum age of 12 years (females) and 13 years (males); in comparison, Atlantic halibut as old as 40–50 years have been collected elsewhere in the western North Atlantic. Growth modeling confirmed sexual dimorphism, with a larger asymptotic length (L∞) for females (214 cm fork length [FL]) than males (195 cm FL). Estimates of median female length at maturity, L50, of 128 cm FL (124–132 cm, 95% confidence limits), and median female age at maturity, A50, of 9.6 years old (9.0–10.8 years), were longer and older than previous reports for the Gulf of Maine, likely resulting from our use of histological instead of macroscopic methods to classify maturity. Histology demonstrated that vitellogenesis initiated in individuals in spring, nearly a year prior to spawning, which allowed us to identify first-time (primiparous) spawners and provided the first potential evidence of skip spawning for this species. Finally, an index was developed to track the proportion of potentially mature females in the fishery, which showed an increasing trend; this qualitative tool may prove useful in a data-limited environment for evaluating the relative stock status of Atlantic halibut.

Fast Somatic Growth May Cause Recruitment Overfishing in Vendace (Coregonus albula) Gillnet Fisheries

Vendace is commonly assumed at low risk to recruitment overfishing. This assumption has been confirmed for boreal stocks but might not apply at lower latitudes. We evaluated the risk of recruitment overfishing at the southernmost extent of vendace populations by comparing fecundity, natural mortality, and fishing mortality of two stocks differing in growth with those of a population not subject to fishing. Although in one study lake vendace grew faster owing to high lake productivity and low stock density, and had substantially higher age-specific fecundity, intense harvest before first spawning negatively affected the stock's recruitment potential by reducing spawner abundance. In contrast, in the slow-growing stock, vendace entered the fishery after first spawning, and egg production per recruit was similar to that of the population not subject to fishing. We conclude that vendace stocks characterized by fast somatic growth may be at higher risk of recruitment overfishing, which can be reduced by protecting first-time spawners.

Low impact of first-time spawners on population growth in a brown trout population

For species with individual variation in reproductive success, experience in breeding and the distribution of different breeders is important for population productivity and viability. Human impacts, such as climate change and harvesting, can alter this distribution and thus population dynamics. Here, we investigated the effect of spawning experience on population growth in a population of migratory brown troutSalmo truttasubject to stressors including migration barriers, harvesting, and climate change. We described the population dynamics with a structured integral projection model that differentiates between first-time and repeat spawners. We then took a scenario-based approach to test to which extent spawning experience has a positive effect on the population growth of brown trout by running 3 different model simulations: a baseline scenario with no changes to the reproductive output of the population, a non-selective scenario in which the reproductive output of all spawners was reduced, and a selective scenario where the reproductive output of only first-time spawners was reduced. We found that the reproductive output of repeat spawners is more important than that of first-time spawners for population growth, in line with other studies. Moreover, the contribution of first-time spawners to the population growth through their own survival is more important than their contribution to growth through reproduction. To ensure the continued existence of the study population, survival of first-time spawners and reproductive success of repeat spawners should be prioritised. More generally, including breeding experience adds more mechanistic detail, which ultimately can aid management and conservation efforts.

The generalization of Gulland’s method: How to estimate maturity ogives when juvenile data are missing while spawner demography is known

The proportions of mature individuals at age or length, collectively known as the maturity ogive, are a key population characteristic and serve as critical input to age-disaggregated stock assessments. John Gulland showed in 1964 that it is possible to estimate maturity ogives even when representative data on immature individuals are not available, provided that one can distinguish newly mature individuals (first-time spawners) from those that had matured earlier (repeat spawners). Gulland’s method offers a valuable tool for obtaining information on an unobserved part of a population and is also applicable to other ontogenetic transitions, such as metamorphosis, smolting, ontogenetic niche shifts, and sex change. Here we present a full derivation of Gulland’s method from first principles, applicable to the general case in which the survival of immature, first-spawning, and repeat-spawning individuals may differ. Better observation methods, in particular in sclerochronology and histology, are expected to make meeting this method’s data requirements—i.e., the separation of first-time and repeat spawners—more often achievable, and estimating maturity ogives could serve as an additional incentive for allocating resources to enhanced data collection. With the generalization presented here, we hope to make Gulland’s method better known and more widely accessible.

Co-inheritance of sea age at maturity and iteroparity in the Atlantic salmon vgll3 genomic region.

Co-inheritance in life-history traits may result in unpredictable evolutionary trajectories if not accounted for in life-history models. Iteroparity (the reproductive strategy of reproducing more than once) in Atlantic salmon (Salmo salar) is a fitness trait with substantial variation within and among populations. In the Teno River in northern Europe, iteroparous individuals constitute an important component of many populations and have experienced a sharp increase in abundance in the last 20years, partly overlapping with a general decrease in age structure. The physiological basis of iteroparity bears similarities to that of age at first maturity, another life-history trait with substantial fitness effects in salmon. Sea age at maturity in Atlantic salmon is controlled by a major locus around the vgll3 gene, and we used this opportunity demonstrate that these two traits are co-inherited around this genome region. The odds ratio of survival until second reproduction was up to 2.4 (1.8-3.5 90% CI) times higher for fish with the early-maturing vgll3 genotype (EE) compared to fish with the late-maturing genotype (LL). The L allele was dominant in individuals remaining only one year at sea before maturation, but the dominance was reversed, with the E allele being dominant in individuals maturing after two or more years at sea. Post hoc analysis indicated that iteroparous fish with the EE genotype had accelerated growth prior to first reproduction compared to first-time spawners, across all age groups, whereas this effect was not detected in fish with the LL genotype. These results broaden the functional link around the vgll3 genome region and help us understand constraints in the evolution of life-history variation in salmon. Our results further highlight the need to account for genetic correlations between fitness traits when predicting demographic changes in changing environments.

Seasonal timing of reproductive migrations in adfluvial bull trout: an assessment of sex, spawning experience, population density, and environmental factors

Using individual tags combined with a fish fence operated at the mouth of Smith-Dorrien Creek, the primary spawning habitat for Lower Kananaskis Lake bull trout (Salvelinus confluentus), we approximated a complete census of the spawning population from 1996 to 2000 to assess whether timing of upstream and downstream spawning migrations varied with extrinsic and intrinsic factors. The timing of both upstream and downstream migrations varied with sex, previous spawning experience, density, and temperature. Inferred spawning duration based on the predicted upstream and downstream migration dates indicated that experienced female spawners spent the least amount of time upstream and first-time spawners spent the most time upstream. No consistent differences in upstream migration timing were observed between non-repetitive and repetitive spawners. We suggest that variations in spawning migration timing observed in Lower Kananaskis Lake may be linked to environmental factors that influence upstream swimming ability and acquisition and expenditure of energy with respect to reproduction.

Photoperiod manipulation for the reproductive management of captive wolffish populations: Anarhichas minor and A. lupus

This study provides new and practical information to implement the use of photoperiodic manipulation for the control of the sexual maturation of Canadian 6+ Atlantic wolffish (Anarhichas lupus) and 3+ first-time spawners spotted wolffish (Anarhichas minor) from Canadian and Norwegian populations. Wolffish reproductive cycle (gametogenesis and oocyte maturation and spermiation) was monitored for 23 consecutive months. Control groups were held under a 12-month simulated natural photoperiod and treatment groups under an 8-month compressed photoperiod. Plasma sex steroid concentrations (estradiol-17β and 11-ketotestosterone), oocyte diameter growth, and milt production were assessed monthly. For all groups under study, fish subjected to the compressed photoperiod spawned 2–6 months earlier than the controls. Complete out-of-shift cycle was not achieved based on the completion of two reproduction cycles, and this is briefly discussed. Photoperiod treatment induced temporal shifts in sex steroid profiles, which are the likely mediators of altered timing of ovulation/final maturation. Photoperiod has a strong influence on the timing of wolffish maturation and could be used as an efficient and inexpensive tool to secure wolffish reproduction operations (year-round supply of egg and milt and/or timing with optimal temperature regimes).

Assessing reproductive behavior important to fisheries management: a case study with red drum, Sciaenops ocellatus

Spawning site selection and reproductive timing affect stock productivity and structure in marine fishes but are poorly understood. Traditionally, stock assessments measure reproductive potential as spawning stock biomass or egg production and do not include other aspects of reproductive behavior. Red drum make an excellent case study to assess these other aspects, as (1) they are highly fecund, pelagic spawners, like most exploited marine fishes; (2) their life cycle is delineated between nursery (estuarine) and adult (coastal and offshore) habitat; and (3) they are managed at these two spatial scales. This study was conducted from August 2012 to December 2013 and integrates data from multiple methods and spatial scales. Aerial surveys were used for large-scale monitoring of aggregations off two known estuarine nursery areas, Tampa Bay and Charlotte Harbor, Florida, USA. Capture-based sampling in Tampa Bay coastal (n = 2581) and estuarine waters (n = 158) was used to assess reproductive state and to confirm coastal spawning. To assess spatial dynamics, we acoustically tagged two population components in the Tampa Bay system, subadults from the estuary (n = 20) and adults from the coastal spawning site (n = 60). Behavioral plasticity was seen in subadult recruitment to coastal habitat, with some subadults maturing and recruiting before or during the spawning season and others (14 of 20 acoustically tagged fish) recruiting at the end of the 2012 spawning season. Both adults and recruited subadults (n = 29) were consequently detected in the Charlotte Harbor array, 132 km to the south. Spawning-site fidelity to the Tampa Bay spawning site occurred at both the population and individual scales. Aggregations consistently occurred in Tampa Bay coastal waters during the spawning season, and approximately two-thirds of tagged adults returned in the 2013 spawning season. A similar proportion of subadults returned to the Tampa Bay spawning site, exhibiting natal homing. However, these first-time spawners arrived later than repeat spawners and were detected over shorter time periods. This study, and others like it, demonstrates how integrating data from individuals tracked over space and time with more traditional population-based sampling is changing our understanding of ecological processes that affect marine fish productivity and our ability to manage for sustainablity.

Simulation of fishery-induced changes on the reproductive cycle of common sardine, Strangomera bentincki, off central southern Chile

Intense fishing mortality generates changes in marine populations and ecosystems, and an increase in the sensitivity to environmental changes. Fishery-induced demographic changes are the main mechanisms involved due to truncation of age structure of a population trough selective removal of larger and older fish from a population. As a consequence, changes in the seasonal spawning pattern can occur because larger and older fish tend to have a longer and more extensive reproductive activity than smaller and first-time spawners. This is the case of a small pelagic fish, known as the common sardine Strangomera bentincki, which supports an important fish activity off central southern Chile (33°S-41°30′ S). In this paper, fishery-induced changes on the seasonal reproductive cycle of common sardine were simulated through an age-structured population dynamics on a monthly basis. The fishery is dependent on the annual pulse of recruitment and fishing efforts are applied intensively on the juvenile fraction during the first half of each year. Thus, fishing mortality removes fast growing fish or fish recruiting earlier, and thereafter the age structure tends to be truncated, and as a consequence, slow-growing fish or late-recruiting fish could be selected in the population. A delayed peak in the reproductive cycle was observed in the gonadosomatic index during the period 2000–2010, as compared with the period 1993–1999. In addition, a change in the length composition of the catches was observed, supporting the results obtained with simulations. The fishery-induced changes on the reproductive cycle of common sardine can increase the sensitivity of the population to climate variability either matching or mismatching the reproductive cycle with favorable/unfavorable environmental variables for recruitment.

Estimating natural mortality and egg production of snow crab Chionoecetes opilio adult females

The natural mortality rate is a key parameter in ecology and fisheries, but it may be difficult to estimate as it is highly variable and often confounded with other factors such as fishing mortality and migration. This is especially true for crustaceans in general, as age determination is problematic, and for snow crabs Chionoecetes opilio in particular because density-dependent pro- cesses may lead to highly variable mortality rates across life history stages. In this context, we developed an original method to estimate the natural mortality rate of adult (i.e. terminally molted) female snow crab, and its consequences on egg production. This new method relies on shell condition, carapace width and abundance of adult females and was applied to a time series (1991-2012) of annual trawl survey data for a snow crab population in the Gulf of St. Lawrence, eastern Canada. Two natural mortality estimates were provided by the method depending on 2 distinct assumptions about the survey. Both estimates (0.66 and 0.78 yr �1 ) were high compared to previous estimates for snow crab. These values imply that female life expectancy after terminal molt was short and that primiparous females (first-time spawners) contributed a large share (at least 81%) of the total number of eggs produced by the case-study population over the period 1992 to 2010.

Population dynamics of Salmincola salmoneus on Atlantic salmon in a northern Norwegian river

Atlantic salmon Salmo salar are often heavily infected by the gill maggot Salmincola salmoneus, but little information exists on the population dynamics of this parasite. Through a combination of in vivo field examination and laboratory analysis of gills from the Alta River S. salar population in northern Norway, we describe the population dynamics of the parasite and suggest a model for the host-parasite interactions. S. salar did not become infected with S. salmoneus until they returned to the river as first-time spawners. The infection increased rapidly until autumn, and just after spawning 96% of the spent fish (kelts) were infected with a mean intensity of 53 parasites per fish. In May, the prevalence of S. salmoneus on the descending kelts had increased to 100%, but the intensity exhibited little change. A small proportion of the adult S. salar population returned as immature to the river during autumn and had lower parasite intensities than the kelts the following spring. When the fish that had spawned previously (repeat spawners) returned from their second (or more) sea migration, they had an average infection rate of 36 S. salmoneus individuals per fish. The kelts seemed to be the main habitat for the parasite during winter and spring, and they stay long enough in the river to pass the infection to maiden S. salar that enter the river early in summer. These fish then became a source of infection for the maiden fish entering the river later. However, in years that have a possible mismatch between the opposite migration of kelts and maiden S. salar, the immature fall-running and returning repeat spawners will be crucial for maintaining the parasite population. We hypothesize that heavily infected S. salar may suffer reduced growth and survival at sea, potentially reducing the abundance of repeat spawners.

The role of fecundity regulation and abortive maturation in the reproductive strategy of Norwegian spring-spawning herring (Clupea harengus)

This study investigates the reproductive strategy, an important component in the estimation of stock reproductive potential, in Norwegian spring-spawning (NSS) herring (Clupea harengus), an iteroparous, extreme capital spawner, through the estimation of fecundity over a period of 3 years including two complete maturation cycles and three spawning seasons. NSS herring have an ‘optimistic’ strategy, with almost all adult herring caught in August being in the vitellogenic stage of ovary development, despite overwintering energy levels not being determined at this time. Fecundity in the summer, i.e., more than half a year before spawning in spring (February–April), was also much higher than could be supported by an individual’s concurrent energy levels. Consequently, fecundity was later reduced through atresia with the majority of this occurring before overwintering. The total reduction and the length of the time period in which the reduction took place appeared to vary between years. During the spawning season, atresia was mostly prevalent in small first-time spawners <180 g and several individuals aborted ovary development at this time. Final fecundity varied between years with a difference of up to 18% and was linked to annual variations in condition. In conclusion, this extensive field study has demonstrated that each individual herring can display a suite of size-specific reproductive tactics to fine-tune oocyte production in response to fluctuating levels of planktonic prey.

Establishment of new wintering areas in herring co-occurs with peaks in the ‘first time/repeat spawner’ ratio

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 409:189-198 (2010) - DOI: https://doi.org/10.3354/meps08620 Establishment of new wintering areas in herring co-occurs with peaks in the ‘first time/repeat spawner’ ratio Geir Huse1,*, Anders Fernö1,2, Jens Christian Holst1 1Institute of Marine Research, 5817 Bergen, Norway 2Department of Biology, University of Bergen, 5020 Bergen, Norway *Email: geir.huse@imr.no ABSTRACT: Norwegian spring spawning (NSS) herring migrate to wintering areas in late summer and remain there for about 5 mo in dense aggregations. First-time spawning cohorts typically adopt the wintering area of the spawning stock. However, over the last 50 yr the NSS herring stock has occupied 7 discrete wintering grounds, spanning from the high seas to narrow fjords, which shows that there is considerable plasticity in choice of wintering area. We show that 5 cases of establishment of new wintering areas take place when abundant first-time spawning cohorts take up the adult migration pattern. The ratio in abundance between age 4 and age 5+ herring is on average 14 in the years when new wintering areas are established compared to 0.3 in years without changes. The wintering areas vary greatly with regards to temperature and distance to the feeding and spawning grounds, features that heavily affect energy expenditure during wintering and migration. Rather than being strictly optimized, the establishment of new wintering areas seems to be governed by general school cohesion mechanisms when naïve first-time spawners flood the population and the social learning process is disrupted. KEY WORDS: Norwegian spring spawning herring · Wintering area · Demography · Schooling · Migration Full text in pdf format PreviousNextCite this article as: Huse G, Fernö A, Holst JC (2010) Establishment of new wintering areas in herring co-occurs with peaks in the ‘first time/repeat spawner’ ratio. Mar Ecol Prog Ser 409:189-198. https://doi.org/10.3354/meps08620 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 409. Online publication date: June 23, 2010 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2010 Inter-Research.

Defining a spawning population (spotted seatrout Cynoscion nebulosus) over temporal, spatial, and demographic scales

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 394:231-245 (2009) - DOI: https://doi.org/10.3354/meps08262 Defining a spawning population (spotted seatrout Cynoscion nebulosus) over temporal, spatial, and demographic scales Susan K. Lowerre-Barbieri1,*, Nathan Henderson2, Joel Llopiz3, Sarah Walters1, Joel Bickford1, Robert Muller1 1Florida Fish and Wildlife Conservation Commission, Florida Fish and Wildlife Research Institute, 100 Eighth Avenue S.E., St. Petersburg, Florida 33701-5020, USA 2AECOM, 701 Edgewater Dr., Wakefield, Massachusetts 01880, USA 3Rosenstiel School of Marine and Atmospheric Science, Division of Marine Biology and Fisheries, University of Miami, 4600 Rickenbacker Causeway, Miami, Florida 33149, USA *Email: susan.barbieri@myfwc.com ABSTRACT: Fish spawning populations are complex and affected by many factors acting over temporal, spatial, and demographic scales. To better understand these factors, we chose to study spotted seatrout Cynoscion nebulosus, which has closed populations over small spatial scales and a periodic life-history pattern. We used experimental gill nets and hook and line, within a spatially and temporally explicit sampling design, to collect 2034 spotted seatrout in lower Tampa Bay, Florida, USA, in 2001 and 2002. Ovarian development was evaluated histologically. Mature females did not all spawn throughout the spawning season, and first-time spawners (Age 1) had the lowest probability of doing so. Two reproductive behaviors were observed: fish moving in the late afternoon or evening to a high-intensity spawning site in an inlet and low-intensity spawning within the estuary. Larger, older female and male spawners were more abundant at the inlet site than at nearby estuarine sites. Both batch fecundity and the percentage of females spawning increased with age and size. Spatial–temporal interactions made it difficult to assess certain aspects of reproduction, such as spawning frequency. KEY WORDS: Fish spawning · Spawning population · Spatio-temporal · Demographics · Reproduction Full text in pdf format PreviousNextCite this article as: Lowerre-Barbieri SK, Henderson N, Llopiz J, Walters S, Bickford J, Muller R (2009) Defining a spawning population (spotted seatrout Cynoscion nebulosus) over temporal, spatial, and demographic scales. Mar Ecol Prog Ser 394:231-245. https://doi.org/10.3354/meps08262Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 394. Online publication date: November 18, 2009 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2009 Inter-Research.

Effects of female spawning experience and larval size on feeding and growth of cod larvae Gadus morhua L. reared in mesocosms

The viability, defined in terms of growth and feeding success, of cod larvae from first-time and repeat spawners of similar sizes was studied in two mesocosms. The growth rate of larvae from first-time and repeat spawners were similar from hatching through to 41 days old, when the experiment was terminated. Larvae from first-time spawners were heavier throughout the experiment than larvae of repeat spawners, a difference that might be explained by a difference (although non-significant) in egg size between the two groups. There was a significant difference in growth rate between the two mesocosms, which may be explained by the difference in temperature between the mesocosms. In both mesocosms larvae fed mainly on copepod nauplii during the first weeks of life and started to actively select copepodites at age 20 days post hatch. There was no significant difference in the energy content of the ingested prey at a given age between offspring of first-time and repeat spawners, although there was a significant difference between the two mesocosms. The dry mass of the ingested prey items compared to larval dry mass increased until age 17–27 days post hatch and decreased thereafter.

Length-structured analysis of the reproductive season of anchovy and common sardine off central southern Chile

The anchovy (Engraulis ringens) and common sardine (Strangomera bentincki) are two small pelagic fish which have a similar reproductive strategy off central southern Chile. The seasonal reproductive dynamics of both the species was investigated by taking into account the gonadosomatic index (GSI) and visual maturity data as a function of female size for the period 1993–1999, and also the seasonal pattern in condition factor per size classes. Larger females of common sardine (repeat spawners) have a reproductive peak earlier in the season than younger females at first maturity, while larger anchovy have a reproductive peak delayed as compared with first-time spawners. The condition factor of females exhibited an inverse cycle with the seasonal pattern of GSI, and delayed by approximately 6 months. Although larger females tend to present better and wider condition than smaller females, the condition of females seems to be delayed in larger females probably mediated by the seasonal pattern in food availability. This “energy storage strategy” in spring and summer time seems to be size-dependent and past energetic reserves could also affect the egg production and timing of reproduction. We concluded that the reproductive season of anchovy and common sardine is different and mediated by the length structure in the seasonal upwelling system off central southern Chile.

Climate change and condition of herring (Clupea harengus) explain long-term trends in extent of skipped reproduction

It is commonly assumed that iteroparous fish, once mature, normally reproduce in all consecutive seasons. Recent work has suggested, however, that in Norwegian spring-spawning herring-a population that undertakes extensive spawning migrations-almost one in two adults may skip their second spawning migration. Why should herring not return to spawn the year after first spawning, but instead wait an extra year? For herring, participation in distant, energetically costly, and risky spawning migrations will only pay off in terms of fitness if individuals are sufficiently large, and in sufficient condition, to both successfully migrate and spawn. Changes in the environment and individual condition should therefore affect the likelihood of skipped spawning. This paper describes long-term changes in the extent to which the second reproductive season is skipped in this herring population. These are shown to be linked to the size and condition of herring as first-time spawners, and to climatic factors possibly related to food availability. The findings corroborate the hypothesis that skipped reproduction results from trade-offs between current and future reproduction, growth and survival.

Fish breeding in, and juvenile recruitment to, the St Lucia Estuarine System under conditions of extended mouth closure and low lake levels

Ongoing drought conditions along the northern coast of KwaZulu-Natal resulted in the St Lucia Estuary mouth closing, and to evaporation leading to extremely low lake levels and the development of hypersaline conditions in most parts of the system. The mouth closed in June 2002 and has remained closed for more than three years. The St Lucia Estuarine System is an important nursery ground for juveniles of many marine fish species. Ezemvelo KZN Wildlife initiated a once-off survey to investigate the effects of the drought on the fish fauna and to establish if any fish were breeding in the system under the existing drought conditions. Fish were sampled at six sites throughout the system, using seine and gill-nets. In December 2004, the system had become partitioned into four isolated compartments, and at this time a total of 4 475 individuals comprising 30 species were recorded. An analysis of size class data indicated the presence of juveniles of at least 12 species, including nine estuarineassociated marine species, which had spawned after mouth closure. Although some estuarine species had spawned within the system, most appeared to have recruited into the Narrows and lake sections of the St Lucia system during January 2004 when the adjacent Mfolozi River, which is linked to the open Mfolozi Estuary, flooded and overtopped into St Lucia Estuary. Should the St Lucia mouth remain closed for four or more years, it can be expected that the lack of recruitment of first-time spawners from the St Lucia nursery grounds into the off-shore marine breeding stocks will have major impacts on these estuaryassociated marine species.

Life history consequences of overexploitation to population recovery in Northwest Atlantic cod (Gadus morhua)

Changes to life history traits are often concomitant with prolonged periods of exploitation. In the Northwest Atlantic, 30- to 40-year declines of more than 90% of Atlantic cod (Gadus morhua) have been associated with significant reductions in age and length at maturity, changes most parsimoniously explained as genetic responses to fishing. Increased survival costs of reproduction associated with earlier maturity, resulting in higher natural mortality and shorter life span, negatively affect population growth rate and rate of recovery. Coupled with lower hatching rate among first-time spawners and smaller size at maturity, a modest reduction in age from 6 to 4 years can reduce annual population growth in Atlantic cod by 25%–30%, based on the output of a stochastic, age-structured life history model. Earlier maturity more than doubles the probability of negative population growth every generation. These results underscore the potential for fishing-induced changes to life history traits alone to generate slow or negligible recovery in marine fishes, exacerbating negative impacts on population growth resulting from ecosystem-level alterations to interspecific competition and predation.