The species composition, abundance and distribution of the ichthyoplankton of the Guadiana estuary and adjacent coastal area were studied in relation to several environmental parameters. Sampling occurred during new moon Spring tides, at the ebb and flood, at nine stations, from March 2002 to February 2003. Temperature, salinity, dissolved inorganic macronutrients, seston and organic matter and chlorophyll a were determined at each station. A total of 22 and 34 fish eggs and larvae taxa were identified, respectively. The highest abundances were registered in the lower and middle part of the estuary, probably due to the presence of species that use the estuary as a preferential spawning ground, mainly Pomatoschistus spp. and Engraulis encrasicolus sensu lato. Temporal patterns of occurrence of ichthyoplankton allowed each species' spawning season to be determined. Two taxa dominated the ichthyoplanktonic community during spring and summer (Pomatoschistus spp. and E. encrasicolus), whereas Sardina pilchardus was particularly abundant during autumn and winter. The forcing variables responsible for community structure were chlorophyll a and seston for eggs, while temperature, salinity and nitrite explained the larval component. Comparisons of the results with previous ichthyoplankton studies of the Guadiana estuary conducted before the Alqueva dam was built also indicate that river flow has an important impact on the distribution and abundance of ichthyoplankton in the estuary.

Early life stages of many marine organisms are being challenged by climate change, 35 but little is known about their capacity to tolerate future ocean conditions. Here we 36 investigated a comprehensive set of biological responses of larvae of two 37 commercially important teleost fishes, Sparus aurata (gilthead seabream) and 38 Argyrosomus regius (meagre), after exposure to future predictions of ocean warming (+4 ºC) and acidification (ΔpH=0.5). The combined effect of warming and 40 hypercapnia elicited a decrease in the hatching success (by 26.4 and 14.3% for S. 41 aurata and A. regius, respectively) and larval survival (by half) in both species. The 42 length for newly-hatched larvae was not significantly affected, but a significant 43 effect of hypercapnia was found on larval growth. However, while S. aurata growth 44 was reduced (24.8-36.4% lower), A. regius growth slightly increased (3.2-12.9% 45 higher) under such condition. Under acidification, larvae of both species spent less 46 time swimming, and displayed reduced attack and capture rates of prey. The 47 impact of warming on these behavioural traits was opposite but less evident. While 48 not studied in A. regius, the incidence of body malformations in S. aurata larvae 49 increased significantly (more than tripled) under warmer and hypercapnic 50 conditions. These morphological impairments and behavioural changes are 51 expected to affect larval performance and recruitment success, and further 52 influence the abundance of fish stocks and the population structure of these 53 commercially important fish species. However, given the pace of ocean climate 54 change, it is important not to forget that species may have the opportunity to 55 acclimate and adapt.

Critical swimming speed (U crit ) provides a useful estimate of maximum swimming performance for fish larvae that can be used to assess transport and migratory potential. We measured U crit of red drum Sciaenops ocellatus larvae through its ontogeny and compared the swimming performance of laboratory-reared larvae to that of wild-caught individuals. U crit increased with ontogeny (size), even though variability in U crit at any ontogenetic state was large. U crit for wild-caught larvae increased from 9.7 to 22.2 cm s -1 over the range of 8.3 to 16.3 mm TL and from 1.1 to 20.5 cm s -1 over the range of 3.0 to 19.1 mm TL for reared larvae. The ontogenetic increase in critical swimming speed occurred in 2 phases -an early phase of rapid improvement and a later phase of slower improvement. This sharp change in the trajectory of swimming performance coincided with important changes in ecology, morphology, and hydrodynamics. During the early phase, larvae were pelagic, their growth was highly allometric, especially in the caudal region, and they swam in the inertial hydrodynamic regime. The onset of the later phase coincided with settlement into seagrass beds, isometric growth, and inertial effects on locomotion. Wild larvae generally exhibited greater values of U crit than reared larvae of a comparable size, but the difference was not statistically significant. The results of this comparison imply that research on reared larvae may provide naturalistic results for swimming performance and that hatchery-produced larvae may perform certain behaviours well when released into the wild.KEY WORDS: Scaling · Ontogeny · Swimming performance · Settlement · Hydrodynamics · Wild larvae · Reared larvae Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 384: [221][222][223][224][225][226][227][228][229][230] 2009 important question is whether the behaviour, or specifically swimming performance, of reared larvae is similar to that of wild larvae. From the available data, the answers are mixed. It is generally agreed that behaviours with a learned component, such as anti-predator behaviour, will differ between wild and reared individuals because larvae from the 2 sources have very different experiences (Olla et al. 1998, Brown & Laland 2001). However, some tested behaviours present contradictory results. For example, reared fish may swim faster, slower, or they may have performances equivalent to wild fish, with ontogenetic variations further complicating this picture (von Westernhagen & Rosenthal 1979, Danilowicz 1996, Smith & Fuiman 2004). The same is true for other behaviours. So, when using results from reared individuals, it is desirable to compare the behaviour of these larvae to that of wild fish, especially if the results will be used to make inferences about fish in nature (Leis 2006).Some fishes that breed in coastal marine waters use estuaries as nursery habitat for their late-stage larvae and juveniles. To enter an estuary, larvae may rely on passive transport (J...

The ontogeny of swimming behaviour in sardine Sardina pilchardus larvae was studied, from hatching to 75 days post-hatch (dph), by measuring the critical swimming speed (U crit ) and observing locomotory behaviour. In addition, the effect of larval nutritional condition on U crit at the onset of their swimming abilities (20 to 25 dph) was evaluated by rearing larvae under 4 different feeding treatments. Diets consisted of different concentrations of dinoflagellates, rotifers and the copepod Acartia grani, and a wild plankton assemblage. Recently hatched larvae were mostly inactive, but from 2 dph onwards larvae started to swim freely in the rearing tank, and time spent swimming increased throughout ontogeny. Larvae younger than 20 dph (i.e. < 7.90 mm TL) could not swim for the entire adjustment period at the minimum current speed, but thereafter U crit increased significantly with larval age and length, reaching a maximum of 9.47 cm s −1 at 19.10 mm TL and 55 dph. Growth, survival and the nutritional condition of sardine larvae, assessed by the RNA residual index, were significantly higher for larvae reared with the high-concentration diet, contrary to the other derived nucleic acids indices (RNA/DNA and DNA/DW), which showed no differences between diets. Despite differences in the survival and growth rates of sardine larvae, U crit at the onset of swimming did not differ significantly among diets, but was significantly related to larval nutritional condition as assessed by the RNA residual index. Overall, our results show that early larval stages of sardines have poor swimming ability and probably rely on food patches in the wild to survive; however, close to metamorphosis (especially from 45 dph onwards), larvae spend most of the time swimming and are capable of resisting the mean current speeds of their natural environment, which may strongly enhance chances for survival. KEY WORDS: Ontogeny • Critical swimming speed • U crit • Sardina pilchardus • Nucleic acid derived indices • RNA/DNA • Foraging Resale or republication not permitted without written consent of the publisher

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