No abstract

The natural mortality of exploited fish populations is often assumed to be a speciesspecific constant independent of body size. This assumption has important implications for size-based fish population models and for predicting the outcome of sizedependent fisheries management measures such as mesh-size regulations. To test the assumption, we critically review the empirical estimates of the natural mortality, M (year )1 ), of marine and brackish water fish stocks and model them as a function of von Bertalanffy growth parameters, L ¥ (cm) and K (year )1 ), temperature (Kelvin) and length, L (cm). Using the Arrhenius equation to describe the relationship between M and temperature, we find M to be significantly related to length, L ¥ and K, but not to temperature (R 2 = 0.62, P < 0.0001, n = 168). Temperature and K are significantly correlated and when K is removed from the model the temperature term becomes significant, but the resulting model explains less of the total variance (R 2 = 0.42, P < 0.0001, n = 168). The relationships between M, L, L ¥ , K and temperature are shown to be in general accordance with previous theoretical and empirical investigations. We conclude that natural mortality is significantly related to length and growth characteristics and recommend to use the empirical formula: ln(M) = 0.55 ) 1.61ln(L) + 1.44ln(L ¥ ) + ln(K), for estimating the natural mortality of marine and brackish water fish.

We investigate changes in the North Sea fish community with particular reference to possible indirect effects of fishing, mediated through the ecosystem. In the past, long-term changes in the slope of size spectra of research vessel catches have been related to changes in fishing effort, but such changes may simply reflect the cumulative, direct effects of fishing through selective removal of large individuals. If there is resilience in a fish community towards fishing, we may expect increases in specific components, for instance as a consequence of an associated reduction in predation and/or competition. We show on the basis of three long-term trawl surveys that abundance of small fish (all species) as well as abundance of demersal species with a low maximum length (Lmax) have steadily and significantly increased in absolute numbers over large parts of the North Sea during the last 30 years. Taking average fishing mortality of assessed commercial species as an index of exploitation rate of the fish community, it appears that fishing effort reached its maximum in the mid-1980s and has declined slightly since. If the observed changes in the community are caused by indirect effects of fishing, there must be a considerable delay in response time, because the observed changes generally proceed up to recent years, although both size and Lmax spectra suggest some levelling off, or even recovery in one of the surveys. Indeed, significant correlations between all community metrics and exploitation rate were obtained only if time lags R 6 years were introduced.

We revisit the empirical equation of Gislason et al. (2010, Fish and Fisheries11:149–158) for predicting natural mortality (M, year−1) of marine fish. We show it to be equivalent to , where L∞ (cm) and K (year−1) are the von Bertalanffy growth equation (VBGE) parameters, and L (cm) is fish length along the growth trajectory within the species. We then interpret K in terms of the VBGE in mass , and show that the previous equation is itself equivalent to a −⅓ power function rule between M and the mass at first reproduction (Wα); this new −⅓ power function emerges directly from the life history that maximizes Darwinian fitness in non‐growing populations. We merge this M, Wα power function with other power functions to produce general across‐species scaling rules for yearly reproductive allocation, reproductive effort and age at first reproduction in fish. We then suggest a new way to classify habitats (or lifestyles) as to the life histories they should contain, and we contrast our scheme with the widely used Winemiller–Rose fish lifestyle classification.

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Fisheries management based on catch shares – divisions of annual fleet‐wide quotas among individuals or groups – has been strongly supported for their economic benefits, but biological consequences have not been rigorously quantified. We used a global meta‐analysis of 345 stocks to assess whether fisheries under catch shares were more likely to track management targets set for sustainable harvest than fisheries managed only by fleet‐wide quota caps or effort controls. We examined three ratios: catch‐to‐quota, current exploitation rate to target exploitation rate and current biomass to target biomass. For each, we calculated the mean response, variation around the target and the frequency of undesirable outcomes with respect to these targets. Regional effects were stronger than any other explanatory variable we examined. After accounting for region, we found the effects of catch shares primarily on catch‐to‐quota ratios: these ratios were less variable over time than in other fisheries. Over‐exploitation occurred in only 9% of stocks under catch shares compared to 13% of stocks under fleet‐wide quota caps. Additionally, over‐exploitation occurred in 41% of stocks under effort controls, suggesting a substantial benefit of quota caps alone. In contrast, there was no evidence for a response in the biomass of exploited populations because of either fleet‐wide quota caps or individual catch shares. Thus, for many fisheries, management controls improve under catch shares in terms of reduced variation in catch around quota targets, but ecological benefits in terms of increased biomass may not be realized by catch shares alone.

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The most obvious effect of fishing on non-target species is direct mortality. To quantify this effect on the vulnerability of species requires measurement of the current fishing mortality rate and of the tolerance of the species to fishing mortality. These are difficult to estimate for the little-studied non-target species. We describe two potential methods for estimating current fishing mortality rate when data are limited. Their application is illustrated for dab (Limanda limanda) and grey gurnard (Eutrigula gurnardus), two common non-target species in the North Sea. We also develop approaches to define tolerance levels for fishing mortality for little-studied and rare species, based on the potential jeopardy level: the fishing mortality that causes a reduction in spawning stock biomass per recruit relative to the unexploited situation. We propose that for non-target species, models founded on basic knowledge of life history parameters, and on generally established relationships between these parameters, may offer the only practical approach