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.

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Effort management has been proposed as an alternative for quota management in mixed demersal fisheries. It requires a metric to estimate the fishing mortality imposed by a given quantity of nominal fishing effort. Here, we estimate the partial fishing mortality rate imposed by one unit of fishing effort (Fpue) during individual fishing trips and explore the usefulness of this indicator for managing North Sea beam trawlers >300 hp targeting sole (Solea solea) and plaice (Pleuronectes platessa). Fpue is positively related to vessel engine power, and increased annually by 2.8% (sole) and 1.6% (plaice). The positive trend was due to an increase in skipper skills and investment in auxiliary equipment, the replacement of old vessels by new ones and, to a lesser extent, to upgrade engines. The average Fpue imposed per day at sea by a 2000 hp beam trawler was estimated to be 1.0 × 10−5 (sole) and 0.6 × 10−5 (plaice), and it showed substantial seasonal and spatial variations. The Fpue of sole and plaice were negatively related in summer and showed no relationship in winter. The existence of predictive seasonal and spatial patterns in Fpue opens up the possibility of fine-tuning management by directed effort restrictions and uncoupling management of plaice and sole.