In a multi-gear and multi-species artisanal fishery, the level of technical interactions (i.e. the competitive externalities resulting from a shared exploitation of common resources or fishing grounds) among various fishing units is high. Assessing these technical interactions is of great importance for fishery management, as any control applied to one fishing unit may have positive or negative effects on others. The magnitude and direction of these effects cannot be easily measured, unless all fishing units and species in the fishery are considered simultaneously. Technical interactions are particularly important in the complex artisanal fisheries of the English Channel. Using a bioeconomic model of the English Channel that incorporates all the major fishing units (the BECHAMEL model), we describe a method for measuring and classifying the technical interactions due to the competition for resource (stock externalities). The results are used to develop a typology of métiers and fleets based on their overall level of interaction for the resource. We also define fleets and métiers as structuring, dependent, intermediate or autonomous.

Nephrops norvegicus (Norway lobster or prawn) is an important target species in demersal fisheries of the north-east Atlantic. Trawling for Nephrops is wasteful when many small fish are caught and discarded in the process. Here, data from 106 commercial fishing trips, sampled between 1982 and 1998 as part of the Scottish discard sampling programme, are used to investigate the fish bycatch and discards of Nephrops trawlers in the Firth of Clyde (west of Scotland). A large proportion of the fish caught in the Clyde is discarded, the discards mainly consisting of small demersal fish (mean length about 19 cm), particularly young whiting (Merlangius merlangus). Within the study period, annual estimates of total fish discards ranged from 318 to 3027 tonnes, with a mean of 1761 tonnes. Fish landings and discards biomass per unit effort both decreased over the study period. However, the decline in landings per unit effort was greater than that in discards per unit effort, corresponding to an increase in the discard rate over time. In recent years, discards have comprised about 70 % of the fish bycatch. The mean length of discarded fish was positively related to mesh size.

We studied the reproductive biology of the lesser sandeel, Ammodytes marinus, on the fishing grounds of the east central North Sea and on an unfished ground off the coast of southwest Norway. As in other parts of the North Sea, gonad growth appeared to start in September and spawning occurred in December-January. Based on data from the spawning period, maturity ogives by length and age were derived. The length and age at 50 % maturity was 14 cm and 3.2 years respectively in both study areas. The age estimate is 1 year higher than that found previously in the southern North Sea and adopted for the ICES-assessments of the North Sea spawning stock. Estimates of fecundity appeared higher in the North Sea than in coastal waters, at least for large fish. The coastal water estimates also seemed low compared with fecundity at length relationships previously published from Shetland, Fair Isle, and Dogger bank. The sex ratio seldom deviated substantially from the 1:1 ratio, but in summer there appeared to be a surplus of females. Due to spatial differences in age-structure, the proportion of adults was generally much lower on the fishing grounds of the North Sea proper than on the unfished coastal ground. In the middle of the spawning period, however, aggregations of adults occurred on the North Sea grounds.

The distribution and population structure of the red shrimp, Aristeus antennatus, were investigated for an unexploited fishing ground in the Greek Ionian Sea (middle-eastern Mediterranean). A total of 8618 specimens, 7273 females and 1345 males was caught during a 12-month survey, from January to December 1997, at depths ranging from 400 to 800 m. Monthly size frequency distributions were analysed using different techniques, resulting in estimates of growth and mortality. Carapace lengths (cephalothoracic lengths, CL) ranged between 12 and 62 mm for females and 9 and 45 mm for males. The mean size of specimens exhibited statistically significant differences in relation to sex and months. Recruitment of both sexes occurred in mid-winter (January). Only the mean size of females seemed to increase with depth. Estimation of the von Bertalanffy growth parameters revealed high k values and a life expectancy of 4 and 5 years for males and females, respectively. A high percentage of specimens bore spermatophores during May and August, indicating that these months were peak breeding periods. Natural mortality rates ranged from 0.55 to 0.70 for females and from 0.62 to 0.79 for males, depending on the method used. Based on its biology and population dynamics, it appears that the red shrimp population in the study area has the potential to support a viable fishery.

A total of 361 skipjack tuna (Katsuwonus pelamis) between 41.5 and 71.5 cm fork length and 333 yellowfin tuna (Thunnus albacares) between 48 and 150 cm fork length were collected in the western Indian ocean, between latitude 5° N to 10° S and longitude 45° to 65° E from February 1989 to November 1990. Yellowfin populations have one major reproductive season during the north monsoon while in skipjack the reproduction is almost continuous all year round with two peaks in activity during north and south monsoons. The gonadosomatic indexes (GSI) are correlated with the three climatic situations: the north monsoon, the south monsoon and two inter-monsoon seasons. The steroid hormone variations (oestradiol, oestrone and testosterone) are well correlated with the GSI in females. In males, maximum levels coincided with the two main reproduction periods in skipjack while the maximum values of testosterone and 11-ketotestosterone levels were only found during the north monsoon. Even if both species are continuous spawners, these results indicate the existence of different reproductive traits, especially when considering steroidogenesis and vitellogenesis. These findings suggest that over our fishing zone, the reproductive activity is significantly modulated by climatic changes.

Small-scale fish farming in tropical Africa is mainly based on pond culture of Nile tilapia Oreochromis niloticus with supplementary organic fertilisation. Until recently, the ‘high’ stocking densities practised (2 fish·m–2) led to stunted populations and very low yields. Since 1996, rural fish farming developed significantly in Côte d’Ivoire. Only a dramatic decrease in stocking densities allowed fish farmers to produce marketable-sized tilapia with reasonable yield in extensive culture. Density control is achieved through tilapia monosex male culture and stocking the predator Hemichromis fasciatus. Further development of low-input tilapia farming requires improvement of production results. A growth model based on maximal growth and ingestion, maintenance needs and efficiency of food for growth was developed to explain and help optimise production results. The model was validated with farm and experimental data and demonstrated that, for a given rearing period (from 30–50 g fingerling, to market-size), yield increased with density, but then decreased beyond an optimal density. When market-weight target is set, the same variation of yield according to stocking density is observed. Densities of 2 to 3 fish·m–2 markedly reduce tilapia yield in extensive culture. Moreover, increasing market weight from 150 to 450 g·fish–1 will only induce a limited decrease in maximal yield (around 20 %) if the density is lowered and the rearing period lengthened. This decrease is more than compensated by the higher price of large tilapia. Using rearing cycle data, the model can predict the results of alternate combinations of stocking density and duration of rearing period under the same pond management. Thus, production results can be optimised given the objectives and constraints of each fish farmer. This model is a valuable tool to develop extensive tilapia farming in tropical areas, and raises questions for researchers.

Retention of the second polar body for the production of triploid Arctic charr (Salvelinus alpinus) was induced by 65 MPa (650 bar) pressure shocks applied 30, 40 or 50 min after fertilization, each shock lasting 5 min. Triploid yield (percentage of relative survival to control × percent triploidy) varied from 55 to 100 % and it was generally superior to 80. Despite increased mortality and decreased growth in diploid charrs during the spawning period, growth and survival did not differ between diploid and triploid Arctic charr up to and during their third year of life. Most of the triploid males had developed testis, but they did not render milt; few triploid females had developed ovaries. Fecundity of triploid females was low and fertilized eggs from triploid females did not hatch.

Changes in the total amino acid (TAA) and the free amino acid (FAA) contents during embryonic development, through newly spawned eggs, to pre-settled larvae of blacklip abalone (Haliotis rubra) are described. The TAA (protein bound + free) and the FAA contents increased prior to hatching but decreased towards settlement, but the changes were not always significant between different stages of development. Threonine, arginine, lysine and leucine accounted for nearly 50 % of the total essential amino acids (TEAA) in all developmental stages. The mean FAA content of newly spawned eggs was 262.8 ± 28.2 pmol·ind–1 and accounted for 11.5 ± 8.3 % of the TAA. Free essential amino acid (FEAA) content increased significantly as development progressed (P < 0.05), in which threonine, arginine and lysine accounted for over 63 % of this pool. In all developmental stages, the FAA pool was dominated by the non-essential amino acids taurine + proline which accounted for 79.5 % of the total. Generally, the FAA accounted for between 10 to 15 % of the TAA in the different developmental stages of blacklip abalone. All evidence appears to indicate that in blacklip abalone the energy requirements during early ontogeny are mostly met with from the lipid reserves, and that there is a tendency to conserve amino acids until pre-settlement.