Nils Teichert

Reproductive biology of an amphidromous goby, Sicyopterus lagocephalus , in La Réunion Island

In the Indo-Pacific region, rivers are inhabited by amphidromous gobies. They are often subjected to a heavy fishing pressure, highlighting the urgent need to acquire knowledge on their biology for management purposes. This study investigated the reproductive strategy of Sicyopterus lagocephalus, a widespread amphidromous goby, in two rivers of La Réunion Island. Histological observations of ovarian samples and oocyte-size frequency distributions revealed that females had a group-synchronous ovarian follicle development (i.e., two cohorts of oocytes were distinguished simultaneously in ovaries). Females laid an entire clutch in a unique event (from 14,304 to 232,475 eggs) and then another batch of oocytes was recruited, showing that the annual fecundity is indeterminate. Ovarian growth was isometric at all oocyte development stages showing that the gonadosomatic index (GSI) is a good proxy of reproductive condition. The main reproductive season of S. lagocephalus spanned from early February to May with a narrower range upstream and a wider one downstream. Reproduction activity is mainly restricted by water temperatures in upstream areas, whereas it is likely influenced by female body condition and competition in downstream areas. The variability of reproductive traits leads to size structure variation in stocks of spawning females throughout the year.

Habitat selection in amphidromous Gobiidae of Reunion Island: Sicyopterus lagocephalus (Pallas, 1770) and Cotylopus acutipinnis (Guichenot, 1863)

Freshwater populations of the Indo-pacific region are characterized by a large proportion of amphidromous species. In this paper, we analyse habitat selection by two amphidromous sympatric Gobiidae: Sicyopterus lagocephalus and Cotylopus acutipinnis in Reunion Island. A sampling method using Point Abundance Sampling (PAS) was conducted in 12 rivers. We used mixed logistic models in order to examine the presence probability of species according to location, downstream-upstream gradient, microhabitat variables (depth, velocity and predominant substrata) and presence of conspecifics and sympatric species. Presence probabilities varied between the sampled rivers. We observed a positive attraction between identical developmental stages of both species, which suggests that social interactions or similar preferences for environmental cues influenced their distribution. The presence probabilities of both species’ juveniles decreased from downstream to upstream. We showed that traditional microhabitat variables weakly explained the spatial distribution of both S. lagocephalus and C. acutipinnis, in Reunion Island. We suggest that weak habitat selection for these species is consistent with the amphidromous life style because of the unpredictability of juvenile settlement and the extreme hydrological variations in tropical rivers.

Spawning-habitat selection of an Indo-Pacific amphidromous gobiid fish, Sicyopterus lagocephalus (Pallas)

Freshwater-fish populations of the Indo-pacific region are characterised by a large proportion of amphidromous species. We analysed the spawning-habitat selection of Sicyopterus lagocephalus (Gobiidae: Sicydiinae) within two rivers of the Reunion Island. Reproduction traces are rare events, so a presence-only sampling procedure was performed to collect egg clutches and a random description of available habitat was conducted to generate pseudo-absence data. Logistic models showed a strong selection for microhabitat variables and mesohabitat units. S. lagocephalus selected areas where availability of potential nests was high (small cobble to small boulder) and where interstitial substratum allowed intragravel flow. Our results suggest that the availability of favourable substratum may be a limiting factor, which could result in occupancy of suboptimal areas. Water depth and velocity had less of an effect on the selection of a spawning area. Higher selection for shallow ( 30 cm s–1) geomorphic units, with an optimum for riffle and cascade, suggested that mesohabitat could be a satisfactory approach to quickly assess the availability of spawning areas within a watercourse for expertise studies.

Reproductive biology of an endemic amphidromous goby, Cotylopus acutipinnis, from La Réunion Island

Amphidromous gobies living in the rivers of tropical islands display a high continuous reproductive effort in response to environmental unpredictability. Females are usually able to spawn a large number of eggs over an extended season, which contributes to offset the high larval mortality. The increasing human pressure on the insular aquatic systems threatens these gobies and highlights the urgent need to acquire knowledge on their biology for management purposes. The present study investigated the reproductive traits of the endemic amphidromous goby, Cotylopus acutipinnis, in a river of La Reunion Island. Ovarian histology showed that females were multiple spawners with a group-synchronous ovarian follicle development and an indeterminate fecundity. Females reproduced from 28.6-mm TL and laid their eggs in a unique event (from 3226 to 106840 eggs), with a spawning frequency of ~1 month or less. The drop in reproductive activity coincided with lower water temperatures and a decrease in daylight duration. Evidence suggested that spawning activity was not disturbed by flooding events recorded a few days before sampling. We suggested that the temperature threshold inducing the ovarian growth (18.3°C for C. acutipinnis) could be a determinant factor explaining the specific repartition of gobies throughout the river.