scholarly journals Systematic bias in estimates of reproductive potential of an Atlantic cod (Gadus morhua) stock: implications for stock–recruit theory and management

2006 ◽  
Vol 63 (5) ◽  
pp. 980-994 ◽  
Author(s):  
C Tara Marshall ◽  
Coby L Needle ◽  
Anders Thorsen ◽  
Olav Sigurd Kjesbu ◽  
Nathalia A Yaragina
Keyword(s):  
Egg Production ◽  
Gadus Morhua ◽  
Atlantic Cod ◽  
Reproductive Potential ◽  
Threshold Level ◽  
Systematic Bias ◽  
Stock Status ◽  
Relative Fecundity ◽  
Time Period ◽  
Spawning Stock

Stock–recruit relationships that use spawning stock biomass (SSB) to represent reproductive potential assume that the proportion of SSB composed of females and the relative fecundity (number of eggs produced per unit mass) are both constant over time. To test these two assumptions, female-only spawner biomass (FSB) and total egg production (TEP) were estimated for the Northeast Arctic stock of Atlantic cod (Gadus morhua) over a 56-year time period. The proportion of females (FSB/SSB) varied between 24% and 68%, and the variation was systematic with length such that SSB became more female-biased as the mean length of spawners increased. Relative fecundity of the stock (TEP/SSB) varied between 115 and 355 eggs·g–1 and was significantly, positively correlated with mean length of spawners. Both FSB and TEP gave a different interpretation of the recruitment response to reductions in stock size (overcompensatory) compared with that obtained using SSB (either compensatory or depensatory). There was no difference between SSB and FSB in the assessment of stock status; however, in recent years (1980–2001) TEP fell below the threshold level at which recruitment becomes impaired more frequently than did SSB. This suggests that using SSB as a measure of stock reproductive potential could lead to overly optimistic assessments of stock status.

scholarly journals Egg production of Baltic cod (Gadus morhua) in relation to variable sex ratio, maturity, and fecundity

2002 ◽  
Vol 59 (12) ◽  
pp. 1908-1920 ◽  
Author(s):  
Gerd Kraus ◽  
Jonna Tomkiewicz ◽  
Friedrich W Köster
Keyword(s):  
Sex Ratio ◽  
Egg Production ◽  
Gadus Morhua ◽  
Reproductive Potential ◽  
Relative Fecundity ◽  
Spawning Stock ◽  
Class Strength ◽  
The Difference ◽  
Eastern Baltic ◽  
Baltic Cod

Observed fluctuations in relative fecundity of Eastern Baltic cod (Gadus morhua L.) were related to food availability during the main feeding period and were used to develop a predictive model that explained 72% of the interannual variations in fecundity. Time series of sex ratios, maturity ogives, and relative fecundity were combined with mean weights-at-age and stock sizes from an analytical multispecies model to estimate the potential egg production (PEP). Relationships between PEP and independent estimates of realized daily and seasonal egg production from egg surveys were highly significant. The difference between estimates of potential and realized seasonal egg production was of a magnitude corresponding to the expected loss of eggs as a result of atresia, fertilization failure, and early egg mortality. The removal of interannual variability in sex ratio, maturity, and fecundity on estimates of PEP deteriorated the relationships in all three cases. PEP proved to be superior to spawning stock biomass as measure of the reproductive potential in a stock-recruitment relationship of Eastern Baltic cod. PEP in combination with the reproductive volume explained 61% of the variation in year-class strength at age 2.

scholarly journals Effect of changes in reproductive potential on perceived productivity of three Northwest Atlantic cod (Gadus morhua) stocks

2005 ◽  
Vol 62 (1) ◽  
pp. 65-74 ◽  
Author(s):  
M. Joanne Morgan ◽  
John Brattey
Keyword(s):  
Sex Ratio ◽  
Interannual Variability ◽  
Egg Production ◽  
Gadus Morhua ◽  
Atlantic Cod ◽  
Relative Rate ◽  
Reproductive Potential ◽  
Basic Information ◽  
Northern Cod ◽  
Spawning Stock

Abstract Variation in maturity-at-age, sex ratio, and potential egg production (through changes in length at age) were examined for northern cod (NAFO Division 2J + 3KL), southern Grand Bank cod (3NO), and southern Newfoundland cod (3Ps). All showed significant interannual variability in each stock. Estimates of reproductive potential were produced by sequentially incorporating estimates of proportion mature at age, sex ratio at age, and potential egg production. The estimates of reproductive potential produced by each method were broadly similar, but there were important differences. This leads to differing perceptions of stock productivity, as measured by relative rate of recruitment of a stock and in the spawning stock produced per recruit. These differing perceptions can have a major impact on expectations for the recovery of depleted stocks and the sustainability of various levels of fishing. Efforts should be made to improve estimates of reproductive potential by further exploring the impacts of changes in the spawning characteristics of populations, and by collecting such basic information as fecundity data.

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 Did population collapse influence individual fecundity of Northwest Atlantic cod?

2007 ◽  
Vol 64 (7) ◽  
pp. 1338-1347 ◽  
Author(s):  
J. C. Stares ◽  
R. M. Rideout ◽  
M. J. Morgan ◽  
J. Brattey
Keyword(s):  
Egg Production ◽  
Gadus Morhua ◽  
Atlantic Cod ◽  
Reproductive Potential ◽  
Population Recovery ◽  
Potential Fecundity ◽  
Northwest Atlantic ◽  
Population Collapse ◽  
Variable Nature ◽  
Individual Fecundity

Abstract Stares, J. C., Rideout, R. M., Morgan, M. J., and Brattey, J. 2007. Did population collapse influence individual fecundity of Northwest Atlantic cod? – ICES Journal of Marine Science, 64:1338 –1347. We examined the influence of population collapse on individual potential fecundity and total population egg production (TEP) of three northwest Atlantic cod (Gadus morhua) populations: northern cod (Divisions 2J3KL), southern Grand Bank cod (NAFO Divisions 3NO), and southern Newfoundland cod (Subdivision 3Ps). Fecundity at length increased in conjunction with population collapse for two (3NO, 3Ps) of the three populations. Subsequent moderate population recovery between the 1990s and 2000s in 3Ps was accompanied by a decrease in fecundity at length. A large decrease in fecundity at length for 3NO during the same time period, despite little or no population recovery, coupled with the fact that there was no obvious difference in fish condition between the two time periods, suggested that density-independent factors could be contributing to the changes in fecundity. Use of pre-collapse fecundity–length relationships to estimate TEP in the post-collapse period resulted in underestimation of TEP by as much as 30% in 3NO and 46% in 3Ps, whereas in 2J3KL, TEP was overestimated by as much as 18%. Although the results do not fully support the hypothesis of an inverse relationship between population size and fecundity, they do demonstrate the variable nature of cod fecundity which, if not accounted for, can lead to erroneous perceptions of stock reproductive potential.

Is spawner biomass a sensitive measure of the reproductive and recruitment potential of Northeast Arctic cod?

1998 ◽  
Vol 55 (7) ◽  
pp. 1766-1783 ◽  
Author(s):  
C Tara Marshall ◽  
Olav Sigurd Kjesbu ◽  
Nathalia A Yaragina ◽  
Per Solemdal ◽  
Øyvind Ulltang
Keyword(s):  
Egg Production ◽  
Gadus Morhua ◽  
Atlantic Cod ◽  
Population Analysis ◽  
Reproductive Potential ◽  
Fish Stocks ◽  
Virtual Population ◽  
Individual Scale ◽  
The Individual ◽  
The Relationship

The assumption that spawner biomass is directly proportional to total egg production by fish stocks underlies most spawner-recruit relationships. Despite its importance, this assumption is largely untested. Data describing the reproductive potential of Northeast Arctic (NEA) Atlantic cod (Gadus morhua) were used to test the proportionality assumption for a period showing strong variation in condition and abundance of spawners. At the individual scale, relative fecundity varied with length and condition. This decreases the likelihood that the proportionality assumption is valid. At the stock level, total egg production was estimated (1985-1996) using acoustic estimates of total abundance and demographic information from trawl sampling in combination with year-specific fecundity-length relationships. For NEA cod, spawner biomass estimated by virtual population analysis (VPA) was not proportional to total egg production. Compared with VPA-based estimates of spawner biomass, total egg production is an improved index of recruitment potential because (i) the magnitude of variation observed in total egg production was closer to that observed in recruitment and (ii) the relationship between recruitment at age 1 and total egg production approaches the origin. The results suggest that further progress in explaining recruitment variation will be achieved using more sensitive measures of the true reproductive potential of the stock.

scholarly journals The dangers of ignoring stock complexity in fishery management: the case of the North Sea cod

2008 ◽  
Vol 4 (6) ◽  
pp. 693-695 ◽  
Author(s):  
William F Hutchinson
Keyword(s):  
North Sea ◽  
Egg Production ◽  
Gadus Morhua ◽  
Fishery Management ◽  
Atlantic Cod ◽  
Management Strategies ◽  
Climatic Conditions ◽  
The North ◽  
Spawning Stock ◽  
The North Sea

The plight of the marine fisheries is attracting increasing attention as unsustainably high exploitation levels, exacerbated by more extreme climatic conditions, are driving stocks to the point of collapse. The North Atlantic cod ( Gadus morhua ), a species which until recently formed a major component of the demersal fisheries, has undergone significant declines across its range. The North Sea stock is typical of many, with a spawning stock biomass that has remained below the safe biological limit since 2000 and recruitment levels near the lowest on record. Cod within the North Sea are currently managed as a single stock, and yet mounting empirical evidence supports the existence of a metapopulation of regionally variable, genetically distinct, sub-stocks. Applying the same management strategies to multiple stocks that differ in their resilience to exploitation inevitably results in the overfishing and likely collapse of the weaker components. Indeed, recent studies have identified two North Sea spawning stocks that have undergone disproportionally large collapses with very substantial reductions in egg production. Similarly affected cod stocks in the northwest Atlantic have shown little evidence of recovery, despite fishery closures. The possible implications of ignoring sub-structuring within management units for biocomplexity, local adaptation and ecosystem stability are considered.

scholarly journals Does increased information about reproductive potential result in better prediction of recruitment?

2011 ◽  
Vol 68 (8) ◽  
pp. 1361-1368 ◽  
Author(s):  
M. Joanne Morgan ◽  
Alfonso Perez-Rodriguez ◽  
Fran Saborido-Rey
Keyword(s):  
Reproductive Biology ◽  
Egg Production ◽  
Gadus Morhua ◽  
Reproductive Potential ◽  
Essential Element ◽  
Greenland Halibut ◽  
Greenland Halibut Reinhardtius Hippoglossoides ◽  
Difficult Challenge ◽  
The Relationship ◽  
Hippoglossoides Platessoides

The relationship between stock size and recruitment is an essential element in the understanding of the productivity of a population. However, predicting the number of recruits produced by a population has proven to be a difficult challenge. This may in part be a result of poor estimation of reproductive potential (RP). We determined if including increased information on reproductive biology in indices of RP results in better predictions of recruitment. We investigated some of the conditions that lead to better (or worse) recruitment prediction when more biologically complex indices of RP are used. Data from four populations in the Northwest Atlantic were examined: southern Grand Bank (NAFO Division 3NO) cod ( Gadus morhua ), Flemish Cap (NAFO Division 3M) cod, Newfoundland (NAFO Division 3LNO) American plaice ( Hippoglossoides platessoides ), and Greenland halibut ( Reinhardtius hippoglossoides ) (NAFO Subarea 2 + Division 3KLMNO). Stock–recruit models paired with complex indices of RP gave a better estimate of recruitment in slightly more than half of the tests conducted. When there were larger trends in the reproductive biology (maturity at age, sex ratio and egg production), more complex indices of RP were more likely to provide a better estimate of recruitment.

The role of spawning stock in formation of recruitment of northeast atlantic cod Gadus morhua

2006 ◽  
Vol 46 (1) ◽  
pp. 74-82 ◽  
Author(s):  
V. M. Borisov ◽  
A. A. Elizarov ◽  
V. D. Nesterov
Keyword(s):  
Gadus Morhua ◽  
Atlantic Cod ◽  
Northeast Atlantic ◽  
Spawning Stock

The influence of spatially variable and connected recruitment on complex stock dynamics and its ecological and management implications

2019 ◽  
Vol 76 (6) ◽  
pp. 937-949 ◽  
Author(s):  
Lisha Guan ◽  
Yong Chen ◽  
James A. Wilson ◽  
Timothy Waring ◽  
Lisa A. Kerr ◽  
...  
Keyword(s):  
Gadus Morhua ◽  
Gulf Of Maine ◽  
Atlantic Cod ◽  
Temporal Trends ◽  
Substantial Reduction ◽  
Management Implications ◽  
Agent Based ◽  
Stock Biomass ◽  
Spawning Stock ◽  
The Individual

To evaluate the influence of spatially variable and connected recruitments at spawning component scale on complex stock dynamics, a typical agent-based complex stock was modeled based on the Atlantic cod (Gadus morhua) stock in the Gulf of Maine. We simulated three scenarios with different degrees of connectivity (i.e., individual exchange) between the spatially variable recruitments of 36 spawning components within four subpopulations under the stock. Subsequently, the temporal trends were compared for different scenarios in age-1 recruitment, spawning stock biomass, and local depletion proportion of the overall complex stock and the individual subpopulations. Results show that increased recruitment connectivity from 0.1–0.2 to 0.6–0.8 between various components tends to increase the productivity and stability of a complex stock at local and global scales and reduce the proportion of depleted components due to overfishing. Moreover, depletions of less productive components may occur without a substantial reduction in the overall complex stock biomass and recruitment.

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