An investigation was carried out during the Illex argentinus trawl fishery (January-July) in the fishery regions of 42 °S and 45-47 °S off the Exclusive Economic Zone of Argentina between 1985 and 1990. Statoliths of 5,232 squid were sampled and processed to study age and growth patterns in squid, hatched in different months. Among squid of similar ages (within an age interval from 180 to 300 days), the May-hatched squid were the smallest while the September-hatched were largest. Growth rates of squid of the same hatching-months were significantly different according to year, but these differences were considerably smaller than those observed between various hatching-months within one year. The largest individuals at a given age were captured in 1990, and the smallest in 1985. Males and females became mature at age 220-320 and 210-360 days respectively. The maturation period lasts 1.5-2.5 months in males and 1 month in females. Early and late-maturing groups were observed within each hatching-month. The peak. of spawning was in July of 1984, 1986, 1987, in June of 1985 and in August of 1989.

Time series (1965-1985) of Illex illecebrosus catch and morphometric data from the Northwest Atlantic were analysed to describe geographic variability in population structure. The areas studied were NAFO sub-areas 3 to 6, which range from Newfoundland to the northeastern USA shelf. Population components, reflecting seasonal spawning groups, were identified based on analysis of length frequency data. Components 3 and 4 represent two prominent life cycles: the summer spawners and winter spawners respectively. Components 1, 2, and 5 do not represent different life cycles, but result from the capacity to shift between life cycles by prolonging (or shortening) the life span. The presence of up to five components in the southern area illustrates a life history strategy involving protracted spawning and complex population structure. There was clear geographic variability in annual catch, with fluctuations being most extreme in the most northern area. Annual catch levels in all areas were significantly correlated with the abundance of the winter-spawning component, as represented by the number of squid within samples which belong to component 4. Population structure in the most northem area was simplest and catch levels therefore were most dependent on the highly migratory winter-spawning component. This leads to greater catch variability in the most northern area than in the other areas. The advantages of good feeding conditions may compensate for the risks associated with long-range migrations, especially recruitment failure. Life history strategies involving migratory and non-migratory population components limit the risk of recruitment failure. The overall resultant life history strategy for Illex illecebrosus is one that ensures survival of the species by stabilizing recruitment in at least one (southern) area through protracted spawning, complex population structure and interaction of spawning components.

Two feeding experiments were conducted to evaluate growth and survival rates of African catfish (Heterobranchus longifilis Valenciennes, 1840) larvae fed with different diets: live Artemia nauplii, frozen Artemia nauplii, live zooplankton (Moina micrura), frozen zooplankton (Moina micrura), a dry diet based on yeast powder and beef liver, and a commercial trout starter diet. The larvae were fed in excess, six times per day every 4 hours, from the onset of exogenous feeding up to an age of 14 days. In both experiments the final mean weight and specific growth rate of fish fed live Artemia nauplii were significantly higher than those of fish fed other diets (p > 0.05). Survival rates of fish fed Artemia (live or frozen), Moina micrura (live or frozen) and dry diet did not significantly differ (p > 0.05) and ranged from 79 to 92% and 61 to 73% for experiment 1 and experiment 2, respectively. The commercial trout diet led to significantly lower growth and survival rates in comparison to all other diets tested. Although it led to a lower growth rate than that obtained with Artemia, Moina micrura proved suitable for first feeding of H. longifilis larvae. Locally available in West African waters, this cladocera could constitute a valuable alternative for larval rearing when a shortage of Artemia is expenenced. The results also showed that the artificial feed based on yeast powder and beef liver leads to survival rates as high as those obtained with living diets and thus represents a promising way for feeding H. longifilis fry. However, further studies on the nutritional requirements of the larvae remain necessary in order to improve the efficiency of dry diet in terms of growth performance.

Adult two-year-old broodstock sea bass (Dicentrarchus labrax), raised in captivity, were fed ad libitum a natural diet (trash fish) until the end of their first spawning, and maintained under natural conditions of light and temperature. Six months before the second spawning, fish were randomly separated in two tanks with running sea water (salinity 37.80‰, pH 8.3) and maintained under similar environmental conditions. Fish were fed two different ration levels: group F, 1.04%, and group H, 0.45% body weight/day. The Jow ration supplied affected broodstock growth as indicated by decreased growth rates in weight and condition factor in group H, when compared to group F. The mean fecundity per female (No. eggs/female) in group H fish was similar to that from F group, while the relative fecundity (No. eggs/kg female) was more elevated in the H group. However, group H showed a delay in spawning time and slightly higher number of spawnings per female and larger spawning period, when compared to group F fish. In addition, group H females had reduced 17β-estradiol (E2) plasma levels and presented vitellogenic oocytes in the ovary a month later than group F females. By contrast, the vitellogenin (VGT) plasma levels remained similar between groups over the reproductive cycle. The egg quality (i.e. percentage of buoyant eggs), the egg biochemical composition and larval survival at hatching were similar between the two groups. The larvae from both groups also exhibited the same survival after 40 days of culture, although group H produced smaller eggs that yielded smaller newly-hatched larvae. Although the low ration supplied significantly reduced the growth and E2 plasma levels of sea bass, these preliminary results suggest that these changes do not dramatically affect the egg viability or the survival of larvae 46 days after hatching.

Juveniles of the common dentex (Dentex dentex) (2.4 g mean weight), were grown over a 6 week period, fed exclusively on a commercial sea bream diet or an isocaloric moist pellet diet, which was produced on site. Ambient temperature (24.4-26.2 °C) and oxygen content (4.8-5.9 mg/L), as well as the relatively low stocking density (max. 2.62 kg/m3) provided favourable rearing conditions. Survival (63.8% vs 51.6%), specific growth rates (6.1% vs 4.5%) and food conversion efficiency (115.8% vs 76.3%) were considerably elevated in the group fed on moist pellets. The growth performance parameters recorded are considered to be favourable for aquaculture requirements. Agonistic behaviour (particular biting of the tail) was found to be the main cause of the continuous mortality that occurred in both feed groups (78% and 64.8% of total mortality in the dry- and moist-pellet group, respectively). These biting attacks targeted the smallest individuals in a tank, whose mean individual weight made up 49-62% (with dry pellet diet) and 28.9-46.6% (with moist pellet diet) of the respective mean weights. The results indicate a strong relationship between nutrition, size variation and agonistic behaviour in the rearing of cornmon dentex juveniles.

Characteristics of Lactic Acid Bacteria (LAB) isolated from rotifers, Brachionus plicutilis, resembled those of Lactobacillus plantarum or Carnobacterium sp. It was investigated whether the artificial maintenance of a high concentration of LAB in rotifers might increase their dietary value for turbot larvae, particularly when the fish were infected with pathogenic Vibrio sp. One LAB strain was cultivated, then introduced daily into the enrichment medium of rotifers. In this way, LAB were retrieved in large amounts in turbot, and a significant limitation of larval mortality rate was observed when turbot were challenged with pathogenic vibrio at day 9. The inoculum concentration of LAB had a decisive effect on survival rate, and the optimum was between 107 and 2 × 107 Colony Forming Units (CFU) daily added per ml of the enrichment medium (53% survival rate after 72 h of challenge, versus 8% for the infected control group without LAB).

This paper presents some features of turbot sperm and compares them with data reported in the literature on other freshwater and marine fish. In captivity, turbot spermiation period lasts 6 months. When compared to other fish species, turbot males are characterized by low values of maximum gonadosomatic index (0.6-0.8%) and are poor sperm producers in terms of volume (0.2-2.2 ml), concentration (0.7-11.0 × 109 spermatozoa/ml) and total sperm number released per stripping (0.2-12.0 × 109 spematozoa). Compared to most freshwater species, the duration of cell movement is long (1-17 min). The spermatozoon of turbot can be considered to be of a primitive type. A very high protein content (8.8 mg/ml) is found in the seminal fluid of turbot. Increasing collection frequency of turbot males, from monthly to fortnightly and weekly stripping, results in the release of successive samples presenting decreasing semen volume and spermatozoa concentration. Similar spermatozoa concentration and total volume of semen produced per turbot, during a 2 month experimental period, are released from males submitted to an annual light and temperature cycle, compared to those receiving a 6 month contracted light and temperature schedule.