Relationship between swimming capacities and morphological traits of fish larvae at settlement stage: a study of several coastal Mediterranean species

Links between the timing of life-history transitions and dietary and morphological variation during early life history in the sand goby, Pomatoschistus minutus

Bipartite life histories involve a suite of morphological changes that support the pelagic to demersal transition and an expanded range of prey options and microhabitats. Pelagic individuals are thought to shift (settle) to their preferred benthic habitat at the earliest opportunity once they have attained a minimum level of morphological competency to access their new environment. In theory, early changes in larval morphology (collectively termed 'metamorphosis'), habitat and diet-a measure of habitat-use-ought to be synchronous. Yet relationships may be decoupled by factors linked to behaviour, prey availability or morphological complexity, and few descriptions exist to allow such synchrony to be assessed. The sand goby, Pomatoschistus minutus, is a common coastal fish across north-western Europe, with a size at larval metamorphosis and settlement of around 10 and 16-18 mm standard length (SL), respectively. We sampled shoreline larval and juvenile populations to examine relationships between morphology, diet and life stage. Prey diversity increased with body length; however, dietary change was clearest at 16-18 mm SL, with a reduction in calanoid copepods and shift to larger prey such as Nereis polychaetes and mysid and amphipod crustacea. Early growth in five prey capture and processing morphologies was rapid. Four of these showed a subsequent marked shift to slower growth, but none of these changes were aligned with size at metamorphosis and only that of mouth width coincided with body size at settlement. Early life history in P. minutus appears geared towards a protracted morphological reorganization prior to demersal life and an alternative suite of prey resources. Larval metamorphosis seems to be of limited consequence in this regard. Comparable studies of other Baltic Sea fishes would confirm whether these dynamics relate to shared environmental pressures or to factors intrinsic to P. minutus biology.

Ontogeny of swimming performance of hatchery-reared post-larvae and juvenile fish: a case of two threatened Mediterranean species

Swimming performance is a well-established key physiological parameter of fish that is highly linked to their fitness in the wild. In the context of fish restocking purposes, it therefore appears crucial to study this specific behaviour. Here, the authors investigated intra and interspecies differences in the swimming performance of hatchery-reared post-larvae and juveniles belonging to two Mediterranean candidate threatened species, the common dentex, Dentex dentex (Sparidae), and the brown meagre, Sciaena umbra (Sciaenidae), with body sizes ranging from 8 to 37 mm total length (TL, from 24 to 58 days post-hatch). The swimming abilities were estimated through the calculation of their critical swimming speed (U_crit ), their relative U_crit and their Reynolds number (R_e ). Two different patterns were observed between D. dentex and S. umbra, showing a different effect of ontogeny on the performance of both species. The relative U_crit of S. umbra decreased linearly through ontogeny, whereas the relative U_crit and U_crit of D. dentex increased linearly through the range of sizes tested. The ontogenetic change in U_crit of S. umbra occurred in two stages: a first stage of sharp improvement and a second stage of a slow decrease in performance. Both stages were separated by a breakpoint that coincided with the appearance of a refusal to swim behaviour that occurred shortly after the end of metamorphosis and can potentially be associated with the establishment of this species sedentary behaviour. The swimming performance of both species showed ontogenetic differences. Sciaena umbra had the highest relative performance when its body sizes were the smallest, whereas D. dentex showed the highest relative performance when its body sizes were the largest. These results will be linked to future research on both of these species concerning their escape, exploratory and predatory behaviours, and for restocking purposes to draw a more realistic overview of hatchery-reared juvenile performance. Knowledge of both species' behavioural and swimming performance through ontogeny is important to consider when using hatchery-reared fish juveniles for restocking, as size-at-release can have a large impact on fish survival and thus on restocking success.

Long lasting effects of early temperature exposure on the swimming performance and skeleton development of metamorphosing Gilthead seabream (Sparus aurata L.) larvae

Temperatures experienced during early ontogeny significantly influence fish phenotypes, with clear consequences for the wild and reared stocks. We examined the effect of temperature (17, 20, or 23 °C) during the short embryonic and yolk-sac larval period, on the swimming performance and skeleton of metamorphosing Gilthead seabream larvae. In the following ontogenetic period, all fish were subjected to common temperature (20 °C). The critical swimming speed of metamorphosing larvae was significantly decreased from 9.7 ± 0.6 TL/s (total length per second) at 17 °C developmental temperature (DT) to 8.7 ± 0.6 and 8.8 ± 0.7 TL/s at 20 and 23 °C DT respectively (p < 0.05). Swimming performance was significantly correlated with fish body shape (p < 0.05). Compared with the rest groups, fish of 17 °C DT presented a slender body shape, longer caudal peduncle, terminal mouth and ventrally transposed pectoral fins. Moreover, DT significantly affected the relative depth of heart ventricle (VD/TL_, p < 0.05), which was comparatively increased at 17 °C DT. Finally, the incidence of caudal-fin abnormalities significantly decreased (p < 0.05) with the increase of DT. To our knowledge, this is the first evidence for the significant effect of DT during the short embryonic and yolk-sac larval period on the swimming performance of the later stages.

Modeling larval dispersal for the gilthead seabream in the northwestern Mediterranean Sea

To investigate dispersal and connectivity between spawning and lagoon nursery habitats of the gilthead seabream, Sparus aurata, in the Gulf of Lions (northwestern Mediterranean Sea), we modeled the potential transport of the species' larvae between its supposed main spawning site in the region (the Planier Island) and two of its main local nursery areas (the coastal lagoons of Thau and Salses-Leucate). Passive larval drift simulations using a dispersal biophysical model showed a large variability in the possible trajectories from spawning to nursery areas and in the predicted ages for larvae arrival on the two nursery sites. The most common ages at arrival obtained in the simulations (20-60 days) are broadly consistent with previous modeling studies but contrast with the actual ages of the S. aurata post-larvae collected in 2016 and 2017 at time of the lagoon entrances (60-90 days, from otolith readings). The period between 25 and 70 days being critical for gilthead seabream larvae to acquire sufficient swimming, osmoregulatory, and olfactory abilities to enter coastal lagoons, we argue that ontogenic development plays a crucial role in the transport and local retention of S. aurata larvae in the studied region, explaining the discrepancy between simulation results and observed data.

Swimming Abilities of Temperate Pelagic Fish Larvae Prove that they May Control their Dispersion in Coastal Areas

The Sense Acuity and Behavioral (SAAB) Hypothesis proposes that the swimming capabilities and sensorial acuity of temperate fish larvae allows them to find and swim towards coastal nursery areas, which are crucial for their recruitment. To gather further evidence to support this theory, it is necessary to understand how horizontal swimming capability varies along fish larvae ontogeny. Therefore, we studied the swimming capability of white seabream Diplodus sargus (Linnaeus, 1758) larvae along ontogeny, and their relationship with physiological condition. Thus, critical swimming speed (Ucrit) and the distance swam (km) during endurance tests were determined for fish larvae from 15 to 55 days post-hatching (DPH), and their physiological condition (RNA, DNA and protein contents) was assessed. The critical swimming speed of white seabream larvae increased along ontogeny from 1.1 cm s−1 (15 DPH) to 23 cm s−1 (50 and 55 DPH), and the distance swam by larvae in the endurance experiments increased from 0.01 km (15 DPH) to 86.5 km (45 DPH). This finding supports one of the premises of the SAAB hypothesis, which proposes that fish larvae can influence their transport and distribution in coastal areas due to their swimming capabilities. The relationship between larvae’s physiological condition and swimming capabilities were not evident in this study. Overall, this study provides critical information for understanding the link between population dynamics and connectivity with the management and conservation of fish stocks.