Ichthyology and Fisheries Science

A review of cannibalism is presented with particular emphasis on the role that this phenomenon plays in applied larviculture. Cannibalism is more ubiquitous than previously assumed and its implications under larviculture conditions are significant. The review is focused primarily on the two principal causes of sibling cannibalism. These are genetic and behavioral, the latter being influenced directly by environmental factors. Cannibalism is regarded as an alternative feeding strategy, more likely to be adopted by larvae and early juveniles which are carnivorous, when resources become limiting. Agonistic behavior induced mortality, the causes of which are identical to those of cannibalism, is also dealt with. Experimental results on the effects of several important environmental factors are presented and evaluated, and suggestions are made to reduce mortality rates under larviculture conditions.

The potential impact of sibling cannibalism on the size and population structure of captive populations of Koi carp, Cyprinus carpio L., which were fed only dry foods, was determined. The rate of cannibalism was found to be positively density-dependent, with the highest rate being recorded at initial stocking densities of 40 larvae 1 (33.3% of the fish had been eaten by cannibals by day 52). Two types of cannibalism could be distinguished: type I, where the head of the prey was rejected once the caudal region of the prey had been progressively ingested; type II, where the prey were caught either head-first or tail-first and were ingested completely. The first type was limited to the period from the ninth to the 15th day of rearing, and the second type started on the 18th day. Measurement of the mouth, head and tail allowed the calculation of predicted cannibal -prey relationships. The consequences and implications of coeval sibling cannibalism in culture systems are discussed.

A high rate of sibling cannibalism is one of the principal obstacles in the rearing of larvae and juveniles of the African catfish Clarias gariepinus. This paper examines the underlying behavioural components of agonistic behaviour under culture conditions. Ten day old catfish larvae were stocked at 30 larvae l−1 in three tank designs with different surface areas and equal volumes. Stocking densities were 1.2, 0.6 and 0.3 fish cm−2 bottom surface. Growth did not differ between treatments. The highest mortality over 30 days was recorded at the medium density (0.6 larvae l−1). Fish stocked at this density showed the highest rate of aggression, while there was no difference in aggression between the highest and the lowest stocking densities. The results indicate that stocking density should be at least as high as 1.2 larvae cm−2 bottom surface area to obtain high production at best survival rates.

The effect of dietary protein and energy level on growth rate, nutritional indices and body composition of two size classes of Haliotis midae was investigated. Diets were formulated to contain three levels of protein (24, 34 and 44%), each with three levels of fat (2, 6, and 10%), to produce diets with estimated protein energy (PE) ratios ranging from 14.5–32.0 mg/kJ. The rates of weight gain, nutritional indices and body composition of both the smaller (0.2-1.0 g) and larger (7.0–14.0 g) abalone were significantly affected by the levels of protein and energy in the diets. Moreover, the two size classes of abalone differed in their responses to the diets. Maximum growth was achieved at a higher dietary protein level in the larger abalone (44%) than the smaller abalone (34%). The protein content of the larger abalone soft tissue was higher (ca. 7%) than the smaller abalone. Irrespective of the lipid content, the percentage of protein deposited by the larger abalone was highest in those fed the 34 and 44% protein diets, whereas that of the smaller abalone was highest for the 24 and 34% protein diets. The condition factor (CF) of small abalone was significantly higher for those fed the 24% protein diets in comparison to those fed the 34 and 44% protein diets, whereas the reverse was found in the larger abalone. It was concluded that the larger abalone had a higher protein requirement than the smaller abalone. Diets containing the highest level of dietary fat (10%) produced significantly lower growth rates and efficiencies of protein deposition in comparison to abalone fed the diets containing 6 and 2% fat, however, these trends were more marked among the small abalone size class. In addition, the feed conversion ratio (FCR) of small abalone fed the 10% fat diets was consistently poorer, but this trend was not present among the larger abalone whose FCR improved with an increasing PE ratio. It was concluded that a dietary lipid level of 10% is too high for H. midae to maintain a maximal growth rate. Proximal analysis of abalone fed the experimental diets revealed that the PE ratio of the diet as well as abalone size significantly affected the carcass composition.

Commercial farmers using formulated feeds need to know what ration size to feed their abalone. In two growth trials the effect of temperature and abalone size on growth rate and nutritional indices of abalone fed a dry formulated diet was quantified. Between 12 and 20 °C growth rate and feed consumption increased, protein-efficiency ratio (PER) improved and FCR did not differ significantly. However, between 20 and 24 °C, growth and feed consumption declined sharply, and PER and FCR deteriorated. The condition factor of abalone decreased with increasing temperature. It was concluded that temperatures between 12 and 20 °C are physiologically optimal for H. midae. This range corresponds to the mean sea temperature range experienced by H. midae within its natural range. Feed consumption, expressed as a percentage of body weight per day, decreased with increasing abalone size. A model which predicts feed consumption as a function of temperature, shell length and feed-conversion ratio was developed.

The optimum temperature for the primary nursing of Clarias gariepinus larvae and postlarvae was experimentally determined using two complementary methods. Growth of larvae was monitored at different temperatures ranging from 22 to 33°C. The final temperature preferendum of juveniles was determined in a horizontal thermal gradient. High growth rates were recorded between 25 and 33°C and the best was at 30°C. Size variation of fishes reared at 22°C was significantly greater than those batches reared at higher temperatures. The final temperature preferendum was found to be 30°C which corresponds to the temperature at which highest growth rate was recorded, and it is concluded that this temperature is probably optimal for most physiological processes.

The otolith length and the total fish length of 9 leptocephali, 29 elvers and 51 sub-adult eels were measured. For the 51 eels a significant correlation between otolith and fish length was found. No similar correlation was found for leptocephali and elvers because of their similar total length. It was found that the growth of the otolith from leptocephali and elvers differs from the growth of herring larvae otoliths.