Spawning Stock Size Research Articles

Processes Contributing to Variability in Regional Patterns of Juvenile River Herring Abundance across Small Coastal Systems

Many populations of anadromous herring, (e.g., alewives Alosa pseudoharengus and blueback herring A. aestivalis, collectively referred to as river herring) are in decline. To help understand the various processes influencing their relative abundance, we studied juvenile river herring populations in 11 small, coastal Massachusetts systems. We examined diel and seasonal movements, variation in patterns of abundance, and relationships between juvenile river herring numbers and seven abiotic and biotic factors (stream discharge, pond temperature, habitat availability, pond transparency, pH, food availability, and spawning stock size). Seasonally, juvenile downstream migration peaked in early summer, and most juvenile river herring emigrated between 1200 and 1600 hours. Little or no emigration occurred in late summer when stream channels were often dewatered, although several streams experienced a smaller, more variable emigration peak in the fall. In univariate regressions, stream discharge, pond volume, surface area, depth, transparency, and pH were significantly related to variation in juvenile abundance across systems. Multiple regression models that integrated discharge, volume, and transparency, as well as multivariate models that included abiotic and biotic influences, trophic effects, and system size, explained 32–82% of the variability in juvenile abundance across systems. Thus, stream discharge, pond volume, transparency, pH, trophic effects, and system size contribute to heterogeneity across systems and may influence the abundance of these fish during freshwater residence. Increased understanding of the sources of heterogeneity in movement patterns and causes of variability in abundance across systems can help to implement more effective monitoring protocols, more informed land-use decisions, and improved management of river herring.

RESILIENCE AND CRITICAL STOCK SIZE IN A STOCHASTIC RECRUITMENT MODEL

A stochastic model for fish recruitment is fitted to data after performing an age-structured stock assessment. The main aim is to investigate the relation between safe levels of spawning stock size and fish stock resilience. Resilience indicators, such as stock recovery time and the frequency that a stock is below a critical size, are computed by means of simulation using the fitted stochastic model. The stochastic element of the model describes the early life stage survival of the fish using a nonlinear stochastic Leslie type of matrix. From catch data and fishing mortality rates, the free parameters in the model are estimated by means of a maximum likelihood method. The performance of the maximum likelihood estimation method is tested by means of simulation. The method is applied to data of a halibut population (Hippoglossus stenolepis) in the Southeastern Bering Sea. It turns out that given the fluctuation in recruitment, data of at least 25 consecutive years are required.

Does spawning stock affect recruitment of New England groundfish?

We evaluated the effects of spawning-stock size on the recruitment abundance of 11 groundfish stocks in waters off New England. Empirical patterns of association between spawning biomass and recruitment were investigated using nonparametric approaches. For most stocks, higher recruitment occurred at high stock sizes and lower recruitment at low stock sizes. Survival ratios, indexed by recruitment per unit of spawning biomass, were higher at low stock sizes and lower at high stock sizes-as expected under density dependence. To assess assumptions about recruitment dynamics, we analyzed constant-recruitment and compensatory and overcompensatory stock-recruitment models with uncorrelated and autocorrelated error structures, using Bayesian methods of statistical inference. Although no single model was adequate for all stocks, most groundfish had better than even odds of having density-dependent dynamics, and most had better than even odds of having dynamics with uncorrelated environmental variation. While some geographic and taxonomic differences in potential productivity were apparent, all stocks exhibited compensation in survival ratio at low stock sizes, albeit with substantial variation. These results indicate that conservation measures intended to increase the spawning biomass of New England groundfish will, on average, result in higher recruitment and, eventually, in increased and more stable fishery yields.

The influence of stock structure and environmental conditions on the recruitment process of Baltic cod estimated using a generalized additive model

The recruitment process and its underlying mechanisms are among the most studied phenomena in fisheries ecology. Traditional models estimate fish recruitment assuming a direct relationship with spawning stock size. However, highly variable environmental conditions, feeding conditions, and other factors can influence and complicate the results of a simple linear regression analysis between stock and recruitment. We used generalized additive models (GAMs) to investigate the influence of environmental conditions and stock structure on the recruitment processes of Baltic cod. The interaction between abiotic factors and old spawners (>5+ years) and eggs produced by old spawners were the most significant explanatory variables. Eggs produced by young spawners have a positive impact on cod recruitment only at high levels of reproductive volume, while old spawners' eggs have the highest positive effect at low levels of reproductive volume. Here we show: (i) that the number of Baltic cod recruits is strictly dependent on the age structure of the population; (ii) that interactions between biotic and abiotic factors are crucial in explaining recruitment variability; and (iii) that GAMs are a powerful technique for defining and quantifying the intricate multidimensional relationship between biotic and abiotic variables involved in recruitment processes.

Predicting the recruitment strength of an annual squid stock: Loligo gahi around the Falkland Islands

Two cohorts of Loligo gahi recruit to the fishery around the Falkland Islands, the first in summer and the second in autumn-winter. We investigated factors affecting recruitment to these cohorts using 13 years of data (1987-1999). The first evidence for density-dependent effects on recruitment in a squid population is reported, with very high spawning stock biomass leading to a reduction in recruitment in both cohorts. October sea surface temperature was negatively correlated with recruitment to the second cohort 6 months later, and a linear model explained 66% of the variance in recruitment strength. A model combining sea surface temperature and spawning stock size explained 77% of the variance. Thus, low October temperatures and moderate stock sizes lead to higher recruitment the following year than high October temperatures and high stock sizes. A strong negative relationship was also found between sea surface temperature in May and the timing of recruitment to the first cohort the following January-February, suggesting that higher temperatures lead to faster development of embryos or paralarvae and earlier recruitment to the fishery. A predictive model of recruitment size and timing should enable better management of L. gahi.

A rough guide to population change in exploited fish stocks

Interpreting how populations will change in response to exploitation is essential to the sound management of fish stocks. While deterministic models can be of use in evaluating sustainable fishing rates, the inherent variability of fish populations limits their value. In this paper a probabilistic approach is investigated which avoids having to make strong assumptions about the functional relationship between spawning stock size and the annual number of young fish (recruits) produced. Empirical probability distributions for recruits are derived, conditioned on stock size, and used to indicate likely stock changes under different fishing mortality rates. The method is applied to cod (Gadus morhua) in the North Sea to illustrate how population change can be inferred and used by fishery managers to choose fishing mortality rates which are likely to achieve sustainable exploitation.

Relationships between Annual Variations in Reservoir Conditions and Age-0 Largemouth Bass Year-Class Strength

Variable recruitment has been documented in populations of largemouth bass Micropterus salmoides throughout the species' range. Shoreline electrofishing was used from 1988 to 1998 to assess the magnitude of variations in year-class strength of largemouth bass at the time cohort mean lengths reached 50 mm in Jordan Lake, North Carolina. Concurrent measures of adult stock characteristics, dynamics of larval and juvenile shad Dorosoma spp., lake elevation dynamics, and seasonal air temperatures were used to test published findings concerning factors that influence largemouth bass year-class strength. Catch rates of juvenile largemouth bass varied fourfold from 1988 to 1995. No significant relationships were observed between largemouth bass year-class size and spawning stock size, shad dynamics, or variations in lake elevations. However, late-winter and early spring air temperatures explained a significant amount of the observed variation in catch rates of age-0 largemouth bass. Abundance of yearling largemouth bass in early spring also varied significantly among years and was significantly correlated with differences in age-0 abundance at 50 mm the previous year, suggesting that differences in abundance established early in the life history of largemouth bass play a significant role in recruitment variation. Although relative year-class strength varied predictably with prespawn air temperatures, long-term trends indicated a decline in relative year-class strength over the course of 11 years. Broadscale factors, such as temperature, may introduce variation about the expected mean year-class size of largemouth bass in Jordan Lake, which can be further modified by variations in local conditions.

A comparison of Greenland halibut (Reinhardtius hippoglossoides(Walbaum)) fisheries and distribution in the Northwest and Northeast Atlantic

Abstract Greenland halibut (Reinhardtius hippoglossoides (Walbaum)) are widely distributed over extensive geographic areas of both the Northwest and Northeast Atlantic Ocean with no break in the continuity of the respective distributions. Although the entire Greenland halibut resource in the North Atlantic is genetically homogeneous they mainly comprise a single interbreeding stock in each of the two regional areas investigated. Both stocks show similar distribution patterns over depth. Generally, larger fish become more abundant and smaller fish less abundant in progressively deeper water with peak abundance occurring over a depth range of 400-1000 m. Greenland halibut in the Northwest Atlantic were observed to be most abundant in bottom temperatures mainly between 2 °C and 6 °C compared to 0 °C to 4 °C in the Northeast Atlantic. The fishery for Greenland halibut in the Northeast Atlantic was unregulated until 1992 although since 1995 catches substantially exceed those advised. The spawning stock size re...

Recruitment variation related to fecundity in marine fishes

An understanding of the processes that control recruitment variation is central to explaining the population dynamics of fishes and predicting their responses to exploitation. Theory predicts that interannual variation in recruitment should be positively correlated with the fecundity of fish species, but empirical studies have not supported this hypothesis. Here, we adopt a phylogenetic comparative approach, which accounts for evolutionary relatedness among stocks and species, to investigate this relationship. We calculated the mean fecundity of fishes from 52 stocks at the mean length of maturity and related this to interannual recruitment variation. We found that in 13 of 14 comparisons between stocks or closely related species, the stocks with higher fecundity have higher recruitment variation. This was true whether or not we controlled for spawning stock size. However, when the analyses were repeated using a traditional cross-species approach, which did not account for the evolutionary relatedness of stocks, the relationships were not significant. This is the first empirical study to link fecundity with recruitment variation and suggests that fecundity is an important component of fish stock dynamics.

Models of Growth and Maturity for North Sea Cod

Although North Sea cod do not show the large changes in growth and maturity seen in other North Atlantic cod stocks, there are systematic changes which may affect routine assessments. Using weight-at-age data from annual ICES assessments and the International Bottom Trawl Survey (IBTS), a descriptive model which explains weight-at-age in terms of a conventional von Bertalanffy curve and a year-class effect is developed. The fitted year-class effects are correlated with stock biomass indicating a density dependent effect. IBTS maturity data are examined which show that the proportion of fish mature at age is a function both of weight and age. A life history model is constructed which can explain the qualitative changes in maturity in terms of an optimal age of maturity given expectations of growth and natural mortality. A descriptive model based on age and weight is used to reconstruct the historical series of maturity ogives where no direct observations are available. Using the derived maturities, new spawning stock size estimates are calculated and compared with the conventional assessment. In general, taking into account the changes in growth and maturity for North Sea cod does not appear to substantially alter the estimates of sustainable exploitation rates for the stock.

A sustainability criterion for the exploitation of North Sea cod

The spawning stock biomass of cod, Gadus morhua, in the North Sea has been in decline for many years. There is evidence that at low stock sizes recruitment is reduced. Using non-parametric methods, equilibrium spawning stock biomass and yield curves are constructed, which are consistent with annual observations that show the continuous decline in the spawning stock can be explained by high fishing mortality rates. A new reference point, G(loss), is defined, which corresponds to the stock replacement line at the lowest observed spawning stock size. Recent fishing mortality rates yield replacement lines with a high probability of exceeding G(loss) and therefore indicate there is a high probability of further stock decline and a possibility of collapse. The replacement line is very sensitive to fishing mortality rate which makes managing the stock at high rates of exploitation risky, A reduction in fishing mortality rate by approximately 30-40% is required to ensure the probability of collapse is small.

Patterns in vendace recruitment in Lake Pyhäjärvi, south‐west Finland

A vendace Coregonus albula population has long formed the basis of a flourishing fishery in Lake Pyhäjärvi, south‐west Finland. Between 1971 and 1990, it exhibited variations in year‐class strength generally characterized by a 2‐year cycle of alternating strong and weak year classes, which implied density‐dependent regulation probably due to asymmetrical food competition between age groups. The abundance of piscivorous predators in the lake and the wanning of water after the hatching of larvae in spring together determined the final year‐class strength of vendace. Since 1990, year‐class sizes have remained very low and the 2‐year cycle has been disrupted. The continuously low levels of recruitment could not be explained by an increased larval mortality, although the decline started with 2 successive years with very high mortality of this life stage which reduced the whole spawning stock to a fraction of earlier levels, and consequently led to low larval numbers such that between 1988 and 1996 larval abundance was positively correlated with the spawning stock size. The importance of the spawning stock size to recruitment could also be seen in the whole 26‐year time series as a significant correlation between year‐class sizes with a 2‐year lag. The observed recruitment patterns of the vendace stock in Lake Pyhäjärvi thus show the importance of spawning stock size, intraspecific competition, predator effects and abiotic environmental variation.

Investigation of density dependence in salmon spawner–egg relationships using queueing theory

A model based on queueing theory is presented to explain salmonid spawner–egg relationships caused by redd superimposition. The model incorporates carrying capacity, spawner arrival times, egg protection, preference of potential redd sites and accumulation of waiting spawners. Results show that the relationship described by the Beverton-Holt stock recruitment model produces a greater density dependent effect than just redd superimposition from random spawning. Whereas, the Ricker stock recruitment model produces similar results as the random spawning model at low spawner levels. A queueing model of waiting spawners explains reduced recruitment at high spawner levels similar to the Ricker model, but results are dependent on the assumptions made about the reduction in egg viability as spawning is delayed in waiting spawners. These results show, that when the carrying capacity and maximum rate of recruitment are fixed, the Ricker and Beverton-Holt models differ at both low and high spawner stock size. Therefore, the choice between Ricker and Beverton-Holt models should not depend only on the comparison of the comparison of recruitment at high spawning stock sizes. The model presented here is a good initial building block for a full life history model for salmonids or other species and similar techniques could be used for the other life history stages.

A comparison of methods for stock assessment of the Norway lobster, Nephrops norvegicus, in the Firth of Clyde

The abundance and biomass of the Firth of Clyde Nephrops norvegicus stock was estimated using two fishery independent techniques, back-calculation of spawning stock size from larval surveys and production estimates, and burrow counts from an underwater television survey. Larval production estimates were available from two separate investigations in different parts of the Firth of Clyde in 1984 and 1990. The annual larval production estimates from the two studies were divided by a relative density weighted mean fecundity (allowing for ovary resorption and egg loss during incubation) to estimate the numbers of spawning females. Length, weight and sex ratio data were then used to estimate the total stock biomass. The results from the two studies were combined to provide an overall estimate for the Firth of Clyde, of approximately 11,100 tonnes. Counts of burrows, obtained from television camera surveys in 1993, were used to estimate N. norvegicus densities and abundance and the latter value, multiplied by mean weight of individuals from trawl samples, gave a stock biomass estimate of about 8000 tonnes for the Firth of Clyde. The estimates from the two techniques compare reasonably well with recent analytical estimates based on fishery data from the same area. Trawl sampling at the same locations chosen for the TV survey indicated a positive log-linear relationship between burrow density and trawl catch rate suggesting the possibility that, with careful planning, trawl surveys could also be used for stock surveys. There was also a positive log-linear relationship between TV counts of emerged N. norvegicus and burrow density suggesting that the proportion of the population out of their burrows and available to trawling was relatively greater at high densities.

Varlation in migration and abundance of norwegian spring spawning herring (Clupea harengusL.)

Abstract The Norwegian spring spawning herring has undergone great fluctuations in abundance and changes in migration. In the middle of this century the stock was in a productive phase, after which a serious depletion took place and the stock collapsed by the end of the 1960s. The primary factor for the collapse was high fishing pressure. A gradual, but small increase in the adult stock took place between 1974 and 1986. In this period the adult herring did not leave the coast after spawning to feed in the Norwegian Sea, as in previous years, but remained near shore to feed and later migrated into the fjords for wintering. The only numerous year class between 1970 and 1987 was that of 1983. When this year class became mature a significant increase in the spawning stock took place. At the same time a change in the migration pattern was recorded. From 1989 spawning extended southwards along the Norwegian coast, and during the 1990s the Norwegian Sea was increasingly reutilized for feeding. However, wintering still took place in the fjords. Two strong year classes, those of 1991 and 1992, started to recruit to the spawning stock in 1994 and 1995. The subsequent three year classes are weak and a decline in the spawning stock size is expected after 1997–98. Without agreement on total allowable catches (TACs), which limit fishing mortality to a low level, the spawning stock may again decrease below the minimum biological acceptable limit, and the potential largest fish resource in the North Atlantic may again be depleted.

Comment: Recruitment paradigms for fish stocks

Over the last 20 years, there has been a substantial shift in the importance most fisheries scientists ascribe to recruitment overfishing; it was widely assumed (Gulland 1983) that most marine fishes could not be recruitment overfished, and the greatest emphasis in fisheries management was on yield per recruit. However, a large number of fish stock collapses that showed reduced recruitment at low spawning stocks led to a general reappraisal, so that Hilborn and Walters (1992) stated that ithe most important and generally most difficult problem in biological assessment of fisheries is the relationship between stock and recruitment.i Myers et al. (1994) used a large data base on many marine fishes to confirm that most stocks showed reduced recruitment at low spawning stock. Gilbert (1997) offers a revisionist interpretation of the Myers et al. (1994) data and appears to threaten to turn back the clock to a time when fishery managers believed they did not have to worry about maintaining spawning stock biomass. Is this truly a backwards step? I have to answer no; Gilbertis paper is a positive step in our ongoing scientific investigation of stock and recruitment because Gilbert highlights the chicken-or-the-egg dilemma in most spawner recruit analysis. Did low spawning stock lead to low recruitment, or did low recruitment lead to low spawning stock? The growing recognition of the importance of environmental changes in fish recruitment (Francis and Hare 1994) emphasizes the difficulty in interpreting the causes of declines in recruitment, and we are essentially revisiting the ThompsonnBurkenroad debate of 50 years ago (Thompson 1950; Burkenroad 1948; Skud 1975) with more data in hand. Gilbertis paper challenges those of us who have advocated maintaining spawning stock biomass as a prudent form of management to tighten up our arguments. We need to develop statistical analyses that determine the relative probability of competing hypotheses about the cause of recruitment declines. The analysis of Myers et al. (1994) did not consider an environmental change hypothesis; Gilbertis analysis is of limited utility because it relies on acceptance and rejection of null hypotheses, a statistical format that is of no utility for decision making. While Gilbert makes us face the alternative hypothesis that recruitment declined because of environmental conditions, he stops short of the next step and asking what is the prudent management action given the uncertainty regarding the cause of the recruitment decline. If we accept that both a recruitment overfishing hypothesis and an environmental change hypothesis are consistent with an individual stockis history, then what should a manager do if faced with a stock showing both reduced recruitment and reduced spawning stock: continue fishing on the assumption that the environment changed, or reduce fishing in an effort to rebuild spawning stock size? This is a problem in statistical decision theory that requires understanding the probabilities of competing hypotheses and a manageris analysis of risk. Clearly, the costs of not reducing fishing pressure at low abundance, if the true cause is recruitment overfishing, are grave: the stock will simply fail to recover until fishing pressure is reduced. The cost of reducing fishing pressure if the cause of the decline is environmental is simply short-term lost catch. These options are summarized in Table 1: the prudent form of management would seem to be to act as if recruitment overfishing were the cause. However, the choice of management option depends upon the probabilities assigned to the alternative hypotheses and the risk preference of the decision maker. The challenge to fisheries science is now to develop methods to determine the probabilities of alternative hypotheses and perform the risk analysis.

Reference points for optimal fish stock management: A lesson to be learned from the Northeast Arctic cod stock

In the North Atlantic the Icelandic, the North Sea, and the Newfoundland cod stocks are currently overexploited. Overexploitation also characterised the Northeast Arctic cod stock, but effective management measures introduced in 1990 and the years thereafter have brought this stock within safe biological limits. The Northeast Arctic cod stock is transboundary and shared between Norway and Russia. As guidelines for a sound management strategy of this cod stock in the future, reference points for management are discussed. As a point of departure, a management strategy which fulfils the objectives for fishery policy stated by Norway is analysed. These objectives, focusing on sustainable harvesting, increased profitability and the role of the fishery as employer of labour in rural districts are fundamental in most of the world's fishery nations. The “optimal” strategy is defined as the one which fulfils these in the best way possible. The natural variations are discussed and the biological and economic yield's dependence upon the rate of exploitation are analysed. The analysis shows that the size of the spawning stock should be no less than 500 000 tonnes and that highest yield is obtained through a rate of exploitation of around 17–30% (equivalent to a fishing mortality of about 0.20–0.40). A spawning stock size of 500 000 tonnes should therefore serve as a “limit reference point” and a fishing mortality of about 0.20–0.40 should serve as a “target reference point” in the management of Northeast Arctic cod. The method described may be applied to other demersal stocks to help establish target and limit reference points in order to conduct a sound management.

Causal correlation between recruitment and spawning stock size of central Baltic cod?

A biologically sensible model for cod recruitment in the central Baltic is proposed. The model takes into account, in a simple way, (1) the spawning stock size of cod, (2) the annual spawning conditions in terms of salinity and oxygen content, and (3) the predation on cod eggs by sprat. The parameters are estimated using regression analysis methods. All three factors are shown to be statistically significant, although sprat predation is the least clear effect. The model residuals are not correlated (no auto-correlation). The model is successfully evaluated against independent data. According to the model, cod recruitment at age 0 is proportional to the spawning stock biomass (SSB) for realistic ranges of SSB, for a given set of spawning condition and sprat predation. It is further shown that it is important to use recruitment data from an MSVPA, where cod cannibalism is included, in stock recruitment analysis of central Baltic cod.

Interannual fluctuations in recruitment and growth of the sardine, Sardinops melanostictus, in the sea of Japan and adjacent waters

AbstractRecruitment and growth of the sardine Sardinops melanostictus fluctuated markedly in the Sea of Japan and adjacent waters between 1978 and 1993. Stock size was calculated using Virtual Population Analysis and average body length in each age class was determined by the number of annual rings on the scales. There is an inverse correlation between average water temperature at a depth of 50 m in the coastal area of the mainland of Japan in winter (January to March) and recruitment R defined as the number of individuals at 1 year old. There is also an inverse correlation between spawning stock size E and reproductive success in (R/E). A multiple regression model using spawning stock size and water temperature in winter as independent variables can explain 73% of variance in reproductive success. It suggests that both density‐dependent and density‐independent factors perform important roles determining reproductive success. There is an inverse correlation between body length and stock size and this suggests that there is a density‐dependent effect on the growth of the sardine.

On a structural time series method for estimating stock biomass and recruitment from catch and effort data

Structural time series or state-space methods are well suited to stock assessment, where catch and effort are readily available observations, and population size is an unobserved component that one would like to describe. We discuss the use of such a model in estimating absolute abundance and recruitment from catch data and an index of abundance. We discuss the model in the context of the time series literature and the earlier fisheries models for such data. Model fitting is via maximum likelihood and the Kaiman Filter; the approach has advantages over traditional models in that it allows for both stochastic population dynamics and sampling error in index measurement. Further, it makes no assumptions about density-dependent growth or any functional relationship between recruitment and spawning stock size. We illustrate the method with an investigation of performance using simulated data and two sets of data pertaining to cod ( Gadus morhua) in the North Sea. In the real examples the age-composition of the catch is available for part of the duration of the series. Virtual population analysis can therefore also be used; the results are found to be compatible with those from a time series approach.