The state of exploitation of the Norway lobster stock, <i>Nephrops norvegicus</i>, in the Central North Sea

The state of exploitation of the Norway lobster stock, Nephrops norveqicus, in the Botney Gut-Silver Pit area (central North Sea) was assessed by analyzing the long-term trends in the average carapace length of the Nephrops landed by Belgian trawlers. These investigations showed that the Nephrops stock in the central North Sea is not overexploited. Apparently, however, the discarding practice on the Nephrops trawlers has slightly change over the years. Especially in 1982 and 1983 this resulted in a short-term but distinct decrease of the average size of the Nephrops in the smallest market class

Reproduction and seasonal behaviour of the Norway lobster, <i>Nephrops norvegicus</i> in the Central North Sea

The landings per unit effort of the Nephrops fishery show a clear seasonal pattern, attributed to the seasonal behavior of male and female Nephrops in general and to the typical behavior of the berried females, which hide most efficiently in their burrows throughout the egg bearing period. The data on the sex ratios of the Nephrops landings are discussed in relation to the reproductive cycle and the seasonal behavior of both male and female Nephrops. During winter time, when most of the females are berried and therefore less accessible to trawling than the males, more than 80 ppt. of the landings are composed of males. During summer time, when the females are as accessible to trawling as the males, the proportion of females in the landings may increase to more than 60%, depending on the size of the Nephrops in question

Effect of population structure, sampling strategy and sample size on the estimation of length frequency distributions and biological parameters: a case study on brown shrimp (<i>Crangon crangon</i>)

In field studies on brown shrimp (Crangon crangon), catches containing large numbers of animals are a common feature, and sub-sampling has become a common practice. So far, however, the cost-benefit-ratio of sampling strategies and sample sizes (in terms of work-load, on the one hand, and statistical soundness of the estimated population parameters, on the other) has never been thoroughly investigated. The present study tries to solve this problem, by means of simulations on theoretical "populations" (= the combined catches of a shrimp cod-end and its cover) with known size distributions, under various conditions with respect to their size composition, and the way they are subdivided into catch fractions (cod-end cover, discards and landings). As far as optimum sample size is concerned, the results of the simulations showed that, in general, samples of 750 animals (all catch fractions combined), will give sound estimates of the mean size of the population, and reasonable estimates of its length-frequency distribution, provided that the original estimates of the numbers-at-length are smoothed with a moving average of order 5. For statistically sound estimates of the size distribution, the total number of measurements has to be increased to at least 1500. With respect to sampling strategy that is to be preferred (samples of a fixed size or proportional samples, weighted according to the relative share of each catch fraction in the population), the outcome of the simulations was much less conclusive. Because of the differences in adequacy between the two methods that were tested, the choice of the optimal sampling strategy will depend on: the kind of information one expects to obtain from the samples; the levels of precision one is aiming for; the size structure of the population in itself; and the way the population is partitioned over the various catch fractions

The Belgian mollusk fisheries

Bivalves and gastropods landed in Belgium are taken as a by-catch of fisheries targeting finfish or crustaceans, and the nation has never had specialized molluscan fisheries. Species landed consist mainly of whelk, Buccinum undatum; and scallops, Pecten maximus and Chlamys spp. Long-term trends in the landings of mollusks are closely related to long-term changes in fleet and vessel size, gear types used, fishing grounds and target species, and demand on the local market. In the 1950' s, the annual landings of whelk fluctuated between 80 and 200 t, while those of scallop were insubstantial. Since then, the landings gradually rose to record figures of 678 t for whelk and 762 t for scallop in the mid-1970's. Most recently, there has, however, been a substantial drop in the landings of both species. Provisional figures for 1991 and 1992 revealed an almost 50% decrease in landings for whelk, and 80% decrease for scallop. Documented history on mollusk trade and consumption in the area nowadays called Belgium goes back to the early days of the Roman occupation. A regional seafood specialty in Belgium is "moules frites" (blue mussels, steam boiled with a mixture of vegetables, and served with French fries). In 1990, over 30,000 t of mussels were consumed, all of which were imported, mostly from the Netherlands. Whelk is eaten locally, but other mollusk species, such as scallops, venerids, and cardiids, were not eaten to any extent until recent decades. Mollusks, as a whole, contribute about one-fifth to the nation's per capita consumption of finfish and shellfish