C. Tara Marshall

Total lipid energy as a proxy for total egg production by fish stocks

The indeterminate relationship between the total biomass of mature fish (spawner biomass) and the number of offspring produced (recruitment) has puzzled population dynamicists and impeded fisheries management. The relationship assumes that spawner biomass (in tonnes) is proportional to the total number of eggs produced (TEP) by the stock, an assumption under increasing challenge. Most stocks require proxies for TEP because contemporary and/or historical fecundity data are lacking. Here we show a positive association between recruitment and the liver weights of spawners in the Barents Sea cod stock which suggests that recruitment is constrained by the amount of lipid energy stored in the liver. This stimulated our interest in estimating total lipid energy (TLE; in kilojoules) for mature females in the stock. We examined the suitability of TLE as a proxy through correlation and simulation analyses. The results indicate that TLE is proportional to TEP and exhibits a similar response to varying food abundance. Replacing spawner biomass with more accurate measures of reproductive potential is essential to developing a rational basis for stock conservation. Correctly specifying the first-order maternal effect on TEP is a prerequisite to detecting environmental and ecological effects on recruitment.

The Role of Fisheries-Induced Evolution

In their Policy Forum (“Managing evolving fish stocks,” 23 November 2007, p. [1247][1]), C. Jorgensen et al. propose evolutionary impact assessments (EvoIAs) as a general tool for managing evolving resources. The basis for their proposal is that fisheries-induced evolution (FIE) is the most

Deriving condition indices from standard fisheries databases and evaluating their sensitivity to variation in stored energy reserves

To evaluate interstock differences in condition, it would be advantageous to develop stock-level condition indices from standardized databases on weight and length. This study describes a method fo...

An evaluation of recruitment indices for Arcto-Norwegian cod (Gadus morhua L.)

Abstract Abundance indices for Arcto-Norwegian cod (Gadus morhua L.) at various life stages were analysed to determine the index that provides the earliest reliable prediction of year-class strength. The indices considered are an egg abundance index; an early juvenile (approximately 3-months old) abundance index; 0-group (age 4- to 5-months) abundance indices; bottom trawl and acoustic survey abundance indices for 1-, 2- and 3-year-old cod, and VPA estimates of the abundance of 3-year-old cod and of spawning stock biomass. Based on a regression analysis, a cohort’s relative abundance as early juveniles is the best early indication of its abundance as 2- and 3-year-olds.

Tracking the signal in year-class strength of Northeast Arctic cod through multiple survey estimates of egg, larval and juvenile abundance

Abstract Egg and larval abundances for Northeast Arctic cod estimated annually from 1959–1993 were combined with estimates of spawning stock biomass (SSB) and stage abundance estimates for the larvae, pelagic juveniles and demersal juveniles to determine when a correlation with recruitment could first be detected and to track how the correlation changed with increasing age/stage. The largest increase in explanatory power occurred going from SSB to the egg stage suggesting that the transition from potential to realised egg abundance is a critical stage and/or that SSB is a poor index of the potential total egg production. Egg stage abundance was significantly correlated with recruitment; however, the portion of the recruitment signal explained by this index differed from the portion explained by subsequent stages suggesting that the signal in year-class strength undergoes modification between the egg and larval stages. Contrary to the expectation that the signal in year-class strength should become stronger as the penultimate recruitment stage is approached, the significance of the correlations with recruitment decreased for the larval and juvenile stages relative to the egg stage suggesting that the measurement error for these latter stages is high. Temperatures during the post-settlement period explained a significant portion of the residual variation in stage abundance/recruitment relationships for the larval, pelagic juvenile and demersal juvenile stages. A multivariate recruitment model was therefore developed and tested using independent data for the periods preceding and following the model calibration period.

Resilience and tipping points of an exploited fish population over six decades

Complex natural systems with eroded resilience, such as populations, ecosystems and socio-ecological systems, respond to small perturbations with abrupt, discontinuous state shifts, or critical transitions. Theory of critical transitions suggests that such systems exhibit fold bifurcations featuring folded response curves, tipping points and alternate attractors. However, there is little empirical evidence of fold bifurcations occurring in actual complex natural systems impacted by multiple stressors. Moreover, resilience of complex systems to change currently lacks clear operational measures with generic application. Here, we provide empirical evidence for the occurrence of a fold bifurcation in an exploited fish population and introduce a generic measure of ecological resilience based on the observed fold bifurcation attributes. We analyse the multivariate development of Barents Sea cod (Gadus morhua), which is currently the worlds largest cod stock, over six decades (1949-2009), and identify a population state shift in 1981. By plotting a multivariate population index against a multivariate stressor index, the shift mechanism was revealed suggesting that the observed population shift was a nonlinear response to the combined effects of overfishing and climate change. Annual resilience values were estimated based on the position of each year in relation to the fitted attractors and assumed tipping points of the fold bifurcation. By interpolating the annual resilience values, a folded stability landscape was fit, which was shaped as predicted by theory. The resilience assessment suggested that the population may be close to another tipping point. This study illustrates how a multivariate analysis, supported by theory of critical transitions and accompanied by a quantitative resilience assessment, can clarify shift mechanisms in data-rich complex natural systems.

Assessing consistency of fish survey data: uncertainties in the estimation of mackerel icefish (Champsocephalus gunnari) abundance at South Georgia

Mackerel icefish (Champsocephalus gunnari) is a semi-pelagic finfish species inhabiting shelf areas in the Southern Ocean. The population at South Georgia is currently exploited by pelagic trawlers fishing close to the seabed. Annual catches peaked at 150,000 t in 1983 and have declined since the mid-to-late 1980s. Bottom-trawl surveys have been conducted since 1987, providing a time series of abundance and size distribution for use in assessing the status of the stock and setting quotas. Food web models suggest that estimates of the biomass from survey data are substantially lower than the amount of icefish required by the local ecosystem. The aim of this study was to assess the uncertainty around current estimates of density and variance, using alternative nonparametric stratified bootstrapping methods. The stratified rescaling bootstrap estimator was identified as the most appropriate method of those tested: in comparison with the existing method, confidence intervals and the inter-annual variability of the estimates were reduced. Numbers-at-age were estimated from mixture distribution models fitted to length-disaggregated density data in order to determine whether individual cohorts were consistently detected by the surveys. Estimates of numbers-at-age could not consistently delineate cohorts in successive years indicating that survey-based estimates of density were biased. These biases may have arisen because the trawl gear did not select individuals of all sizes equally, or because sampling was restricted to the demersal component of the stock. Estimates of abundance of the pelagic component of the stock should be derived from acoustic data to improve the assessment.

Shifts in spawning phenology of cod linked to rising sea temperatures

Warming temperatures caused by climate change have the potential to impact spawning phenology of temperate marine fish as some species have temperature-dependent gonadal development. Inter-annual variation in the timing of Atlantic cod (Gadus morhua) spawning in the northern North Sea, central North Sea and Irish Sea was estimated by calculating an annual peak roe month (PRM) from records of roe landings spanning the last three decades. A trend towards earlier PRM was found in all three regions, with estimates of shifts in PRM ranging from 0.9 to 2.4 weeks per decade. Temperatures experienced by cod during early vitellogenesis correlated negatively with PRM, suggesting that rising sea temperatures have contributed to a shift in spawning phenology. A concurrent reduction in the mean size of spawning females excluded the possibility that earlier spawning was due to a shift in size structure towards larger individuals, as large cod spawn earlier than smaller-sized individuals in the North Sea. Further research into the effects of climate change on the phenology of different trophic levels within the North Sea ecosystem should be undertaken to determine whether climate change-induced shifts in spawning phenology will result in a temporal mismatch between cod larvae and their planktonic prey.