No abstract

The bigeye cigarfish (Cubiceps pauciradiatus) is a small pelagic nomeid of the tropical world ocean, often recovered in the stomach contents of top predators such as tunas, billfishes and marine mammals. In the Indian Ocean, a few studies have investigated the biology and the ecology of this species that is one of the most abundant fish of the intermediate trophic levels. In this paper, we investigated the spatial distribution of C. pauciradiatus in the Indian Ocean using pelagic trawl catches carried out between 20˚N and 45˚S, and the importance of bigeye cigarfish in the diet of 9 piscivorous fishes sampled by different fishing gears in the western part of the Indian Ocean. The highest densities were observed along the eastern coast of Africa and in the Arabian Sea (87 000 individuals per square nautical mile) during the SouthWest Monsoon and in the eastern part of the Seychelles archipelago (62 200 ind. square nmi) during the NorthEast Monsoon. Small sized bigeye cigarfish (20-80 mm SL) was a regular and abundant prey (20 to 200 ind. per stomach) for schooling predators exploited by purse seine fishery such as large yellowfin and bigeye tunas chasing prey near the sea surface. Large sized bigeye cigarfish (61-150 mm SL) occurred in small numbers (3 to 20 ind. per stomach) in the stomach contents of swordfish and of large yellowfin and bigeye tunas caught by longline sets at great depths. Large concentrations of bigeye cigarfish occurred in zones of high productivity, and that species constituted seasonally a strong link in the transfer of energy from low to high trophic levels in this part of the Indian Ocean.

The food composition of the longnose lancetfish (Alepisaurus ferox Lowe, 1833) was studied in two areas belonging to different biogeographic provinces of the western Indian Ocean: Indian Monsoon Gyres Province (MONS) and Indian South Subtropical Gyre Province (ISSG). A total of 158 lancetfish were sampled during two research longline cruises in 1986-1987: 50 within the MONS, from the western equatorial area (between the EEZs of Kenya and Seychelles) and 108 within the ISSG, from the northern part of the EEZ of Mauritius (Saya-de-Malha Bank-Agalega Islands area). In the equatorial area, 135 prey items (of 18 families or higher taxa) were found in the lancetfish stomachs. In the waters of Mauritius, 476 prey items of 53 taxa were recorded. Regional variability in the feeding habits reflects province-specific differences in the prey abundance and the structure of pelagic communities of the intermediate trophic levels. Large predatory crustaceans dominate in the waters of MONS, while they are absent in the poor waters of ISSG. Foraging success of the lancetfish on non-evasive prey was 3.2 times higher in the MONS than in the ISSG. In overall, swimming crab, Hyperiidea, conspecifics, barracudina, hatchetfish, hammerjaw, and Polychaeta were the predominant prey of the lancetfish. Great differences in the food composition of 'small' (FL < 100 cm) and 'large' (FL ≥ 100 cm) lancetfish were recorded. For large lancetfish, the cannibalism rate and the occurrence of large evasive prey and Sargassum algae floating at the sea surface were greater than for small lancetfish.

Conspecific predation among longnose lancetfish Alepisaurus ferox was investigated in four spatio-temporal strata of the western Indian Ocean. The cannibalism level varied from 0 to 45.5% by frequency of occurrence and was negatively related with abundance of non-evasive prey (such as crustaceans Charybdis smithii and Natosquilla investigatoris) and foraging success. Predation by lancetfish is often described as a non-selective process, constrained by local prey availability and by its feeding speed during an attack of prey. Our results show that lancetfish may adapt its opportunistic foraging behavior, feeding on non-conspecific abundant prey such as crustaceans when available, and switching to a high level of conspecific predation in poor waters.

Background. Atmospheric pressure is one of the most important factors affecting fish activity, but the potential influence on migration patterns of fish lacking a gas bladder is unclear. In this study, we analysed the influence of atmospheric pressure on round goby, Neogobius melanostomus (Pallas, 1814), from the coastal waters of the Black Sea. Material and methods. Gobies were maintained at low pressure, decreased by 1333 Pa (= 10 mmHg), for several hours a day at temperature intervals of 10-12°С, 14-16°С, 18-20°С, 22-24°С, and 26-28°С for 5 days and subsequently the pressure was returned to 1333 Pa and the results were compared to controls without pressure/ temperature changes. Results. We found that increasing the atmospheric pressure at temperatures 10-24°C influenced the activity of round goby, which then stabilised within one hour. Fish activity then decreased till it matches the level observed before experimental pressure changes. Changes in atmospheric pressure did not influence fish activity at higher temperatures of 26-28°C. Conclusions. We intended to determine the influence of atmospheric pressure and water temperature on the swimming activity of round goby. The association between the atmospheric pressure, thermal tolerance, and the fish activity levels is suggested as a possible mechanism driving migrations of the round goby.