Improving the stock-recruitment relationship in Icelandic cod (Gadus morhua) by including age diversity of spawners

1998 ◽  
Vol 55 (6) ◽  
pp. 1372-1377 ◽  
Author(s):  
Gudrun Marteinsdottir ◽  
Kristjan Thorarinsson
Keyword(s):  
Gadus Morhua ◽  
Extended Period ◽  
Shannon Index ◽  
Age Composition ◽  
Stock Size ◽  
Age Diversity ◽  
Spawning Stock ◽  
Stock Recruitment ◽  
Mature Fish

The size of the Icelandic cod stock has been gradually declining since the middle of this century. Recruitment has been poor over an extended period of time and much below the long-term average since 1985. Except for the concurrent decrease in stock size and recruitment during this period, the stock size - recruitment relationship is weak. This relationship is improved by including the age composition of the spawning stock. Spawning stock age diversity in each year from 1955 to 1992 was estimated with the Shannon index using the number of mature fish in each age group. By including information on age composition, 31% of the total variation in recruitment was accounted for by the model with stock size, age diversity, and the interaction between the two, compared with less than 15% by single factor models of either age diversity or stock size. These results indicate that age diversity is an important component in stock-recruitment models and that one of the management goals for fish species should be to maintain high age diversity in the spawning stocks.

Potential effects of maternal factors on spawning stock–recruitment relationships under varying fishing pressure

1999 ◽  
Vol 56 (10) ◽  
pp. 1882-1890 ◽  
Author(s):  
Beth Scott ◽  
Gudrun Marteinsdottir ◽  
Peter Wright
Keyword(s):  
Gadus Morhua ◽  
Reproductive Output ◽  
Atlantic Cod ◽  
Reproductive Potential ◽  
Fishing Pressure ◽  
Age Composition ◽  
Standard Population ◽  
Stock Biomass ◽  
Spawning Stock ◽  
Stock Recruitment

The use of spawning stock biomass as a direct measure of reproductive potential may not be valid because of age- or size-specific differences in fecundity and the effect of maternal size and condition on offspring viability. In this study, we examine the potential significance of these effects using modelled Atlantic cod (Gadus morhua) populations. We quantify how changes in the age composition of the spawning stock, due to a range of fishing pressures and under different stock-recruitment relationships, could influence the reproductive output. Quantitative comparisons were made between a "standard" population where all age-classes only suffer natural instantaneous mortality (M = 0.2) and populations that suffer increasing levels of fishing pressure (F = 0.0-1.0). The results of the modelling exercise suggests that if the effects of the loss of more fecund older/larger individuals in the population are not considered, the number of potential recruits produced by populations under higher levels of fishing mortality could be overestimated by as much as 60%. When age/size-related maternal effects on egg viability are also considered, the amount of potential recruits can be overestimated by a further 10% in the heavily exploited populations.

scholarly journals Long-term changes in trophic patterns of Iceland cod and linkages to main prey stock sizes

2011 ◽  
Vol 68 (7) ◽  
pp. 1488-1499 ◽  
Author(s):  
Ólafur K. Pálsson ◽  
Höskuldur Björnsson
Keyword(s):  
Gadus Morhua ◽  
Atlantic Water ◽  
Type I ◽  
Feeding Response ◽  
Total Consumption ◽  
Stock Size ◽  
Main Prey ◽  
Northern Shrimp ◽  
Long Term Changes

Abstract Pálsson, Ó. K., and Björnsson, H. 2011. Long-term changes in trophic patterns of Iceland cod and linkages to main prey stock sizes. – ICES Journal of Marine Science, 68: 1488–1499. Data on the diet composition of Iceland cod (Gadus morhua) were collected in March of the years 1981–2010 and in autumn of the years 1988–2010. Capelin, northern shrimp, and euphausiids dominate the diet in all years and may be classified as the stable food of Iceland cod. Overall, total consumption by the smallest cod (20–29 cm) remained stable over the three decades, whereas that of larger fish has declined since the mid-1990s. This decline may explain the reduced growth rate of cod in recent years. Long-term, prey-specific patterns were identified in consumption, and significant trophic links were found between cod consumption and stock sizes of capelin and northern shrimp. In March, the correlation between cod consumption on capelin and capelin stock size was highly significant, a type I functional feeding response, but not significant in autumn. The correlation deteriorated in the early to mid-1990s and in the early 2000s. Increased inflow of Atlantic water into north Icelandic waters, and associated changes in capelin distribution, may have contributed to this trend. The interaction between cod consumption on northern shrimp and shrimp stock size showed a highly significant type I functional feeding response in both seasons.

Population Dynamics of the Arcto-Norwegian Cod

1967 ◽  
Vol 24 (1) ◽  
pp. 145-190 ◽  
Author(s):  
D. J. Garrod
Keyword(s):  
Gadus Morhua ◽  
Barents Sea ◽  
Fishing Effort ◽  
Catch Per Unit Effort ◽  
Group Iii ◽  
Spawning Stock ◽  
Stock Recruitment ◽  
Post War ◽  
The One ◽  
Definition Of

By reason of its geographical distribution, the Arcto-Norwegian cod (Gadus morhua) supports three distinct fisheries, two feeding fisheries in the Barents Sea and at Bear Island–Spitsbergen, and a spawning fishery off the Norway coast. In the past this diversity of fishing on the one stock has made it difficult to unify all the data to give an overall description of post-war changes in the stock. In this contribution three modifications of conventional procedures are introduced which enable this to be done. These are: (i) a system of weighting the catch per unit effort data from each fishery to a level of comparability; (ii) a more rigorous definition of the effective fishing effort on each age-group; (iii) a method of estimation of the effective fishing effort on partially recruited age-groups.Using these methods the analysis presents the effects of fishing on each fishery in the context of its effect on the total stock, and at the same time it indicates ways in which factors other than fishing may have influenced the apparent abundance of the stock. The treatment of the data is also used to derive estimates of spawning stock and recruitment of 3-year-old cod for subsequent analysis of stock–recruitment relationships.

scholarly journals An explicit solution for calculating optimum spawning stock size from Ricker’s stock recruitment model

PeerJ ◽  
2016 ◽  
Vol 4 ◽  
pp. e1623 ◽  
Author(s):  
Mark D. Scheuerell
Keyword(s):  
Explicit Solution ◽  
Model Fitting ◽  
Maximum Sustainable Yield ◽  
Reference Points ◽  
Stock Size ◽  
Points Of Interest ◽  
Spawning Stock ◽  
Stock Recruitment ◽  
Explicit Formulae ◽  
Recruitment Model

Stock-recruitment models have been used for decades in fisheries management as a means of formalizing the expected number of offspring that recruit to a fishery based on the number of parents. In particular, Ricker’s stock recruitment model is widely used due to its flexibility and ease with which the parameters can be estimated. After model fitting, the spawning stock size that produces the maximum sustainable yield (SMSY) to a fishery, and the harvest corresponding to it (UMSY), are two of the most common biological reference points of interest to fisheries managers. However, to date there has been no explicit solution for either reference point because of the transcendental nature of the equation needed to solve for them. Therefore, numerical or statistical approximations have been used for more than 30 years. Here I provide explicit formulae for calculating bothSMSYandUMSYin terms of the productivity and density-dependent parameters of Ricker’s model.

Pronounced long-term juvenation in the spawning stock of Arcto-Norwegian cod (Gadus morhua) and possible consequences for recruitment

2008 ◽  
Vol 65 (3) ◽  
pp. 523-534 ◽  
Author(s):  
Geir Ottersen
Keyword(s):  
Time Series ◽  
Gadus Morhua ◽  
Experimental Studies ◽  
Reproductive Capacity ◽  
Fish Stocks ◽  
Stock Biomass ◽  
Clear Link ◽  
Spawning Stock ◽  
The Mean

The oldest and largest individuals are disappearing from many fish stocks worldwide as a result of overexploitation. This has been suggested to impair recruitment through decreasing the reproductive capacity of the spawners and increasing the mortality rate of the offspring. By using a time series on spawners biomass by age class for Arcto-Norwegian cod (Gadus morhua) from 1913–2004, I have documented pronounced changes in the spawning stock, including a trend towards younger fish, a less diverse distribution across ages, and a declining proportion of repeat spawners. Despite the total spawning stock biomass (SSB) being at similar levels now as in 1933, the mean age in the SSB has declined from 10–12.5 years to 7–8 years during the study period, and the percentage of fish of age 10 or above in the SSB has decreased from ~97% to ~10%. Contrary to earlier theoretical and experimental studies, no clear link between age structure and recruitment was found here. Recruitment to the Arcto-Norwegian cod stock may thus be more robust towards spawner juvenation than expected, possibly because of strong recruitment compensation.

scholarly journals The logic of skipped spawning in fish

2006 ◽  
Vol 63 (1) ◽  
pp. 200-211 ◽  
Author(s):  
Christian Jørgensen ◽  
Bruno Ernande ◽  
Øyvind Fiksen ◽  
Ulf Dieckmann
Keyword(s):  
Gadus Morhua ◽  
Atlantic Cod ◽  
Gonad Development ◽  
Fishing Mortality ◽  
Skipped Spawning ◽  
Stock Recruitment ◽  
Mature Fish ◽  
Future Gain ◽  
Sexually Mature ◽  
Shed Light

That sexually mature fish skip reproduction, especially in response to poor condition, has been documented in many species. We present results from an energy-allocation life history model that shed light on the underlying logic of skipped spawning, based on the Northeast Arctic stock of Atlantic cod (Gadus morhua). The model predicts that skipped spawning is a regular phenomenon, with up to 30% of the sexually mature biomass skipping spawning. Spawning should be skipped if the expected future gain in reproductive output, discounted by survival, more than balances the expected reproductive success the current year. Skipped spawning was most common (i) among potential second-time spawners and (ii) early in life, (iii) when fishing mortality at the spawning grounds was high, (iv) when fishing mortality at the feeding grounds was low, (v) when natural mortality was low, and (vi) when the energetic and mortality costs associated with migration and spawning were high. Cod skipped spawning more often when food availability was both increased (opportunities for better growth) and decreased (too little energy for gonad development), and this pattern interacted with mortality rate. We conclude that skipped spawning may be more widespread than appreciated and highlight potential consequences for the understanding of stock–recruitment relationships.

Escapement goal analysis and stock reconstruction of sockeye salmon populations (Oncorhynchus nerka) using life-history models

2008 ◽  
Vol 65 (10) ◽  
pp. 2269-2278 ◽  
Author(s):  
Robert B. Lessard ◽  
Ray Hilborn ◽  
Brandon E. Chasco
Keyword(s):  
Life History ◽  
Sockeye Salmon ◽  
Model Estimate ◽  
Policy Variable ◽  
Optimal Harvest ◽  
Spawning Stock ◽  
Maximum Sustained Yield ◽  
History Of ◽  
Stock Recruitment

We compare life-history models with the Beverton–Holt approach of escapement goal analysis. We model the life history of a sockeye salmon ( Onchorhynchus nerka ) population from a spawning stage, through juvenile and adult stages, and ending with adults that return to spawn. We fit models to data by statistically comparing predicted and observed numbers of four dominant adult ages. Posterior estimates of parameters from Markov chain Monte Carlo simulations are then used to assess optimal harvest policies. We search for policies that produce the highest average yield. We find that it is possible to detect density dependence with a life-history model where analysis of Beverton–Holt stock–recruitment relationship fails to do so. We find that Beverton–Holt relationships produce policies and long-term yield estimates that are inconsistent with empirical trends. Conversely, we find that optimal spawning stock sizes and maximum sustained yield estimates using the life-history model estimate are consistent with the historical behavior of fisheries examined. Adding smolt data to the analysis does not substantially change predicted optimal spawning stock size, but decreases the variance in estimated posterior parameter distributions and policy variable distributions.

Simplified Calculation of Optimum Spawning Stock Size from Ricker's Stock Recruitment Curve

1985 ◽  
Vol 42 (11) ◽  
pp. 1833-1834 ◽  
Author(s):  
Ray Hilborn
Keyword(s):  
Stochastic Variation ◽  
Simple Modification ◽  
Recruitment Curve ◽  
Stock Size ◽  
Spawning Stock ◽  
Stock Recruitment ◽  
Simplified Calculation

The optimum spawning stock size for a Ricker stock recruitment curve was shown to be accurately approximated by the equation Ps = Pr(0.5–0.07a) when 0 < a < 3. A simple modification was also shown to incorporate stochastic variation about the stock recruitment curve into calculations of optimum stock size.

Aspects of spawner-recruit relationships, with particular reference to crustacean stocks: A review

1993 ◽  
Vol 44 (4) ◽  
pp. 589 ◽  
Author(s):  
N Caputi
Keyword(s):  
Fisheries Management ◽  
Environmental Variables ◽  
Short Term ◽  
Stochastic Variation ◽  
Spawning Stock ◽  
Stock Recruitment ◽  
The Impact ◽  
Western Australian ◽  
Crustacean Fisheries

The fundamental biological issue for fisheries management is undoubtedly the prevention of recruitment overfishing, i.e. to prevent the spawning stock from being depleted by fishing to a level where it significantly reduces the abundance of recruits. However, for many important fisheries, particularly crustacean fisheries, the stock-recruitment relationship (SRR) is not known. In many cases, research on recruitment has concentrated on short-term studies of recruitment processes to the exclusion of research into the SRR which requires development of long term databases. This paper examines techniques required to model the SRR, using case studies from Western Australian crustacean fisheries. Outlines of potential problems such as errors and biases in the measurement of stock and recruitment indices, the time series nature of the data, and lack of stationarity in the data, are given with possible solutions. Environmental effects, which can greatly influence the abundance of recruits, may need to be determined before the underlying SRR can be seen. Some of the advantages and possible disadvantages of incorporating environmental variables in the SRR are examined. A thorough assessment of SRRs also involves a study of the impact of fishing on the stock and the effect of stochastic variation using simulations. The evaluation of the SRR requires a multi-disciplinary approach which includes the fields of biology, environment, economics, population dynamics and statistics.

Reconstructing historical stock development of Atlantic cod (Gadus morhua) in the eastern Baltic Sea before the beginning of intensive exploitation

2008 ◽  
Vol 65 (12) ◽  
pp. 2728-2741 ◽  
Author(s):  
Margit Eero ◽  
Friedrich W. Köster ◽  
Brian R. MacKenzie
Keyword(s):  
Baltic Sea ◽  
Gadus Morhua ◽  
Atlantic Cod ◽  
Reference Level ◽  
Stock Size ◽  
Intensive Exploitation ◽  
Spawning Stock ◽  
The 1950S ◽  
The Baltic ◽  
Eastern Baltic

The landings of Atlantic cod ( Gadus morhua ) in the eastern Baltic Sea in the early decades of the 20th century were below 50 thousand tonnes and therefore lower than in recent years at very low stock size. These low landings have largely contributed to a perception that the stock size was also low before the 1950s. In this investigation, we demonstrate that cod spawning stock biomass in the years 1925–1944 fluctuated in a similar range as in the periods from the 1950s to the mid-1970s and from the late 1980s onwards and was in most of these years at least twice as high as at present. Fishing mortality before the 1940s was below 0.2, but reached moderate levels during the Second World War. The stock size before the war may be considered as a reference level of biomass at low fishing impact, providing important information for the management of fisheries and the Baltic ecosystem.

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