Etienne Baras

Here, on the basis of published information upon over 800 species from 39 orders and 202 families, we analyse how the size of oil globules in fish eggs varies between reproductive guilds and environments with contrasting salinities. About 80% of marine fishes produce eggs with oil globules, the volume of which (V_O) never exceeds 9% of the egg reserves and averages 2%. These proportions vary very little between marine fish species producing pelagic or demersal eggs. Fewer freshwater fishes (about 40%) produce eggs with oil globules, but their globules are much larger (mean V_O of 10%) and their volumes vary considerably between reproductive guilds: V_O is on average 3–10 times lower in freshwater species with demersal eggs than among freshwater pelagophils and aphrophils (i.e., bubble nesters), where it averages about 40% (range of 23%–69%) of the egg reserves. These results support the idea that oil globules serve to hydrostatic lift in fresh waters, but not or very little in marine waters, where egg buoyancy can be achieved at a lower energy cost through egg hydration. Advantages and constraints of oil globules in fish eggs are discussed in the light of species producing eggs with very large oil globules (V_O > 20%): access to oxygen‐rich water layers that are indispensable for egg development vs. smaller size of larvae at first feeding, since the saturated lipids contained in oil globules essentially serve very little to tissue construction.

El objetivo de este estudio fue evaluar la variación de crecimiento y la supervivencia en larvas de doncella, Pseudoplatystoma punctifer de 1 a 26 días post fertilización (1-26 dpf), cultivadas en situación comunal. Se obtuvieron 04 progenies, las cuáles fueron obtenidas por la combinación de los óvulos de una hembra con un pool de esperma de cuatro machos. Todas las familias fueron cultivadas en un sistema de recirculación de agua y alimentadas con nauplios de artemia a partir de los 3 dpf cinco veces al día y cada 3 horas de 7:30 a 19:30. El promedio de longitud total, viabilidad diferencial y la diferencia máxima en tamaño fueron calculadas para cada familia en las tres etapas (1dpf, 5 dpf y 26 dpf) de muestreo y analizadas por ANOVA. Los resultados mostraron que a partir de las 4 horas después de la eclosión hay una alta contribución de dos familias en la progenie el cuál fue mantenido hasta el final del periodo de seguimiento (26 dpf). Esto es una evidencia directa de la importancia del macho en la historia de vida temprana de la progenie.

Summary: Knowledge of fish migration is a prerequisite to sustainable fisheries management and preservation, especially in large international river basins. In particular, understanding whether a migratory lifestyle is compulsory or facultative, and whether adults home to their natal geographic area is paramount to fully appraise disruptions of longitudinal connectivity resulting from damming. In the Amazon, the large migratory catfishes of the Brachyplatystoma genus are apex predators of considerable interest for fisheries. They are believed to use the entire length of the basin to perform their life cycle, with hypothesized homing behaviours. Here, we tested these hypotheses, using the emblematic B. rousseauxii as a model species. We sampled adults close to major breeding areas in the Amazon basin (upper Madeira and upper Amazonas) and assessed their lifetime movements by measuring variations in ⁸⁷Sr/⁸⁶Sr along transverse sections of their otoliths (ear stones) using laser ablation multicollector mass spectrometry (LA‐MC‐ICPMS). We demonstrate that larvae migrate downstream from the Andean piedmont to the lower Amazon, where they grow over a protracted period before migrating upstream as adults. Contrary to prevailing inferences, not all fish spend their nursery stages in the Amazon estuary. By contrast, the passage in the lower or central Amazon seems an obligate part of the life cycle. We further evidence that most adults home to their natal geographic area within the Madeira sub‐basin. Such long‐distance natal homing is exceptional in purely freshwater fishes. Synthesis and applications. By using otolith microchemistry, we were able to demonstrate a seemingly compulsory basin‐wide migratory life cycle of large Amazonian catfishes. This makes them the organisms performing the longest migrations (>8000 km) in fresh waters. This exceptional life history is already jeopardized by two dams recently built in the Madeira River, which block a major migration route and access to a substantial part of their spawning grounds. Major impacts can be anticipated from the current and forthcoming hydroelectric development in the Amazon basin, not only on the populations and fisheries of this apex predator, but also on Amazonian food webs through trophic cascades.

Individual identification of fish is often desirable for the smallest possible size, but it is crucial that tagging does not interfere with fish survival, physiology, or behavior. We evaluated radio‐frequency identification (RFID) tags (10 mg) and PIT tags (PIT; 32 mg) in fish of two different size‐classes of Nile Tilapia Oreochromis niloticus: 200–500 mg and 640–1,600 mg, wet mass (WM). This produced four categories of tag load for each type of tag: 5.0, 3.3, 2.5, and 2.0% of WM. We tested 30 fish per category. Survival averaged 95.8% for RFID tags and 98.3% for PIT tags. Tag retention after 35 d was 99.1% for RFID tags and 96.6% for PIT tags. Tagged fish grew more slowly than controls. Growth penalty was proportional to tag load, but restricted to the first 4 d after tagging and compensated by catch‐up growth, except in fish <300 mg presumably due to greater difficulties of handling and tagging. Small PIT tags can thus be used confidently in tilapia of about 1.3 g and RFID tags in tilapia of about 0.4 g. If growth is not a premium, the corresponding minimal sizes are 1.0 (for PIT) and 0.3 g (for RFID).

Through the experiments presented here we wanted to test whether egg production of the black-chinned tilapia Sarotherodon melanotheron heudelotii under experimental conditions varies as a function of ambient salinity (fresh waters vs. sea waters vs. hypersaline waters, 0, 35 and 70, respectively) and whether these responses differ between fish acclimated within a few weeks from fresh water to saline and hypersaline environments (experiments E1 and E2, monitoring over 10 and 18 weeks), and individuals born and raised all life long at the experimental salinities (E3, monitoring over 18 weeks). In total, 233 spawns were collected. In each of the three experiments, the reproductive investment (gram of egg per gram of female over 2 weeks) did not differ between salinities of 0 and 35, whereas it was 2–3 times lower at 70 than at 0–35, because of lower spawning frequency (E1-E3), smaller clutch size (E1) and lower spawn mass (E1-E3). Finally, fish acclimated to salinity from fresh water over a few weeks and those maintained at a particular salinity all life long showed similar reproductive traits, thereby emphasizing the remarkable physiological plasticity of this species.

This study relied on the day-by-day analysis of bioenergetics and prey size preference in isolated cannibals of the African catfish Heterobranchus longifilis (13–57 mm standard length, 3–500 mg dry body mass, n = 153) that were offered ad libitum conspecific prey of adequate sizes in small-sized (2-L) environments under controlled conditions (12L:12D, 30 °C). In these conditions, cannibals of increasing body size selected preferentially prey of decreasing size relative to their own, and increasingly closer to the optimal prey size (producing the highest gross conversion efficiency). The role of experience in cannibalism was found of secondary importance relative to body size, both as regards food intake and prey size selectivity. These results indicate that in environments that minimize the escape capacities of their potential victims, as applies to most aquacultural contexts, fish exercising cannibalism tend to forage optimally, which has rarely been evidenced for piscivorous behaviour. The present study further highlights that H. longifilis possesses a very high capacity for growth, which originates from the combination of high food intake and very high conversion efficiency, and makes this species of utmost interest for aquaculturists wherever fast growth is desirable, but also extremely prone to cannibalism wherever feeding schedules are inadequate.