The paper explores the apparent contradiction between a high trawling pressure on juveniles and sustained production of hake that has occurred over the last decade in many Mediterranean fisheries. The practical consequences are followed of assuming rapid declines in natural mortality rate M in the first few years of life to a low, constant adult natural mortality, as well as the observation, for small-mesh trawl cod ends, of declining availability with age. Several approaches are proposed for fitting declining M-with-age with a reciprocal function for hake, using criteria based on mean life-time fecundity, mean age at egg production, existing estimates of adult M, and vectors based on stock productivity assumptions. All vectors of M-at-age were similar to MSVPA estimates of North Sea stocks.

The implications of the changes in mortality with age for stocks harvested by fine-mesh trawls wereexplored in yield per recruit calculations under 2 different hypotheses: 1) using current estimates of growth and adult mortality, 2) M-at-age vectors for juveniles, dropping rapidly from age 0+, and declining availability to trawling for older fish. These hypotheses were compared within yield per recruit analyses. Under the new assumptions, given current F>>M (adults), yield isopleths predict no significant increases in Y/R with stretched mesh > 40 mm, but a substantial decline in fecundity per recruit with small increases in effort by gill nets or longlines, aimed at mature fish. These results are linked to the refugium concept for older fish, and it is speculated that this may be in part responsible for the continued productivity of other sustained fisheries for juvenile resources elsewhere.

Yield-per-recruit models are often used to provide management guidance for the efficient use of a fish cohort and to estimate the biological reference points, such as F0.1 or FMAX. Although model parameters are usually age-specific, these parameters are thought to be more likely related to length than age. In this study, age- and length-structured models are compared in calculating yield per recruit, F0.1, and FMAX at which the yield per recruit is maximized. Using a Monte Carlo simulation approach, I also compare differences in effects of uncertainties of the model parameters on yield-per-recruit analysis and estimation of F0.1, and FMAX using age- and length-structured models. This study shows that differences are small in estimating yield per recruit, F0.1, and FMAX using two types of models when there are no error variances in the model parameters. However, such differences increase with the uncertainties of the model parameters. The yield-per recruit, F0.1, and FMAX estimated using the age-structured model tend to have smaller values and variations compared with those estimated using the length-structured model. This study indicates that although the length-structured model can better incorporate the information observed from fisheries, age-structured yield-per-recruit model can provide estimates of yield per recruit, F0.1, and FMAX, more precisely and more conservatively, and thus is preferred from the conservation viewpoint in fisheries management.

This paper presents a multispecific approach in order to assess the main demersal resources exploited off Senegal (West-Africa). In absence of reliable data on the actual fishing effort, two series of theoretical fishing effort are estimated, using available species abundance indices from 1971 to 1991. These two series correspond to different hypotheses concerning the trends of fleets' fishing power: an optimistic one and a pessimistic one. For each species or taxa, a surplus production model is calculated with landings statistics and effort. The combining of monospecific models provides a multispecific diagnosis. The analysis is completed by the calculation of diversity indices of catches at the equilibrium level, expressed as a function of fishing effort. While considering the uncertainty around this first assessment, we show that Senegalese groundfish resource is globally full or over-exploited, according to the considered hypothesis. Under the pessimistic hypothesis, 9 among the 11 taxa of Pisces were clcarly over-exploited, with fishing effort some-times as much as twice or three times the effort of maximisation (fMSY). The situation is very critical for Arius sp., Epinephelus aeneus, Pagellus bellottii and Pomadasys sp. Using the multispecific approach, pessimistic diagnosis indicates a fishing effort nearly twice fMSY, with a Maximum Sustainable Yield at about 53 000 t for the 11 taxa together. Conversely, in 1991, octopus and cuttlefish appeared under-exploited. The diversity analysis shows that an increase in fishing effort would lead to a decrease in catch diversity. This result is interpreted as a sign of fragility in the exploited ecosystem.

A study of microhabitat use by fourteen abundant fish species was undertaken in the lower Ntem River Basin in Cameroon (Africa). Seventy-one microhabitats including pools, raceways and riffles were studied twice during the 1994 and 1995 dry season. Results obtained by Canonical Correspondence Analysis (CCA) showed a good niche separation along the ordination axes I (λ1 = 0.37) and II (λ2 = 0.20). Microhabitat selection by these species reflects response to several physical factors including width, depth, current velocity and substrate type. However, a substantial part of the variation in species segregation was explained by both instream (% leaves, woods and roots, and aquatic plants) and bankside (% mean canopy closure)cover. A striking pattern encountered during this study was the frequent size-related variation in habitat use displayed by many species. For example, smaller individuals of Brycinus longipinnis, Barbus camptacanthus, Distichodus notospilus, and D. hypostomatus, occurred more often in faster flowing waters than did their larger conspecifics. The association displayed by CCA was high (0.957 and 0.938, respectively for the first and second axis), and Monte Carlo test (99 permutations) revealed that these associations were significantly different from random (p = 0.01). The implications of these results for habitat management are discussed.

Vitellogenin (VTG) levels were measured in the plasma of 143 female skipjack tuna, Katsuwonus pelamis caught in the Western Indian Ocean. These levels were correlated with the gonadosomatic index (GSI) and with plasma 17β-estradiol (E2) levels during the reproductive cycle. VTG and E2 levels were measured using a competitive enzyme-linked immunosorbent assay (ELISA) and a radioimmunoassay (RIA), rcspectively. VTG was purified from a pool of plasma obtained from females with high GSI using a double chromatography method (gel filtration and ion exchange). A specific antibody was obtained in rabbits. The VTG immunoenzymeassay developed gave an assay detection limit (90% binding) of 15 ng.ml-1. Mean VTG levels ranged from 1.2 mg.ml−1 to 4 mg.ml−1. Mean monthly VTG values remained relatively elevated during the resting periods (1.2 mg.ml−1 in April and 1.9 mg.ml-1 in September), while maximum levels reached only 4 mg.ml−1 during the reproductive season. The highest GSI values were observed from November to March during the major reproductive season corresponding to the North monsoon and from early June to late August during the minor reproductive season corresponding to the East monsoon. Lowest GSI values were found in April-May and September-October. Mean E2 levels exhibited considerable variability among all females sampled ranging from 700 pg.ml−1 to 9 ng.ml−1. This study demonstrated that the sexual maturation was correlated with the monsoon seasons and that there was a positive correlation between monthly variations of GSI, VTG and E2 levels in the skipjack tuna population from the Western Indian Ocean.

Long-term effects of constant exogenous ammonia concentrations were investigated in two different batches of turbot juveniles (53 and 73 g) under controlled environmental and feeding conditions. Over the 84-day experiments, ambient un-ionized ammonia (UIA-N, NH3) concentrations were steady (coefficient of variation, 12-16%) and water pH range was 7.88-7.99. Survival was maximum up to 0.33 mg.l−1 UIA-N and at 0.73, 50% mortalities were observed on day 52 (73 g) or day 77 (53 g). No-growth concentration was 0.73 mg.l−1 UIA-N in the two groups and the 84-day lowest-observable-effect oncentration range 0.14-0.33 mg.l−1. In the most tolerant group (53 g), the 84-day no-observable-effect concentration was 0.14 mg.l−1 UIA-N. The estimated 28-day efficient concentration that gave 50% of the specific growth rate of controls ranged from 0.60 to 0.75 mg.l-1 UIA-N. Specific growth rate to ambient ammonia concentration patterns were different in the two groups for the lowest concentrations (0.14-0.34) and similar to food intake to ammonia pattern. Food efficiency was negatively correlated to ambient ammonia concentration, with major changes in food conversion ratio, and protein utilisation observed above 0.33-0.70 mg.l−1 UIA-N. The main change in body composition was an increase in water content in all ammonia-exposed fish. A significant increase in urea-N excretion rate was also observed over 0.33 mg.l−1 UIA-N. Thresholds for growth were affected by exposure duration. Turbot juveniles growth capacity may be depressed by usual ambient ammonia concentrations under intensive farming conditions.

In mollusc hatcheries and nurseries, live microalgae are traditionally used as feed for bivalves. Despite recent improvements in efficiency, such intensive culture of microalgae represents major cost factors in the production of bivalve larvae and spat. In an attempt to provide cost-effective alternatives and to simplify hatchery-nursery procedures, non-living algal food has been developed over the past few years. Substitution products such as bacteria, yeast, vitamin or docosahexaenoic acid enriched powders, dry and concentrated microalgae and microcapsules have been tested and evaluated on an experimental scale. The present article reviews major research aimed at developing non living diets in mariculture, which generally resulted in lower growth and higher mortalities than that recorded for controls fed live microalgae. At the present time, concentrated and dried microalgae appear to be the best alternative products for bivalves, for juveniles essentially, while microcapsules provide an excellent research tool to study their nutritional requirements.