Postflexion Larvae Research Articles

Effects of Rearing Temperature on Growth and Survival of Blackthroat Seaperch, Doederleinia berycoides (Hilgendorf, 1879), Post-Flexion Larvae and Juveniles

In post-flexion larval and juvenile stages of hatchery-reared blackthroat seaperch Doederleinia berycoides (Hilgendorf, 1879), the influence of water temperature on growth and survival was examined to identify the optimal rearing temperatures. Temperature treatments were conducted separately for pelagic post-flexion larvae (32 days post hatching [dph]) and settled juveniles (69 dph). Fish were transferred to four replicate 30 L tanks (30 larvae or 15 juveniles per tank) and reared for 15 days under five temperatures (13, 16, 19, 22 and 25 °C) maintained by using heaters and coolers. The mean survival rates (± SD) of post-flexion larvae were high (>73 %) for all temperatures (13 °C: 73.3 ± 6.9 %, 16 °C: 76.7 ± 6.8 %, 19 °C: 80.8 ± 10.7 %, 22 °C: 79.2.7 ± 2.8 %, 25 °C: 71.7 ± 4.4 %). Growth increased at higher temperatures and was significantly faster at 22 and 25 °C (P < 0.05) than lower temperatures. For juveniles, mean survival rates were significantly higher at 16–25 °C (>90%: 16 °C: 91.7 ± 1.7 %, 19 °C: 95.0 ± 3.2 %, 22 °C: 91.7 ± 1.7 %, 25 °C: 91.7 ± 5.0 %) than at 13 °C (71.7 ± 7.9 %) (P < 0.05). Juvenile growth, like that of post-flexion larvae, was faster at 22 and 25 °C. These results suggest that maintaining a high rearing temperature (22–25 °C) is important for enhancing the growth for post-flexion larvae and juveniles of blackthroat seaperch and reducing the rearing period to the size of release seedlings.

Early life-history stages of the olive barb, Systomus sarana (Hamilton), and a key ontogenetic systematic character of Smiliogastrinae.

The current study aims to uncover the early life-history stages of Systomus sarana, a medium-sized smiliogastrin cyprinid important for aquaculture in South Asia. The fish were effectively bred in captivity by administering 0.25 mL of breeding hormone per kilogram of fish. The spawning occurred 8.54 ± 0.55 h after the injection, and the eggs were phyto-lithophilic with a pale yellow color and a diameter of 1.49 ± 0.04 mm. Hatching occurred 17 h after fertilization, and the yolk-sac larvae of 3.43 ± 0.08 mm total length (TL) were adhering to the plant parts and other substrata with the cement glands on the forehead. On the third day, with complete absorption of the yolk sac and the disappearance of the attachment organ, the pre-flexion larvae measured 5.3 ± 0.11 mm TL. On the eighth day, the flexion larvae measured 6 ± 0.4 mm TL with a well-inflated posterior swim bladder, and the post-flexion larvae, at 11 days post-hatching (dph), developed a two-chambered gas bladder. The juvenile stage, on day 21 post-hatching, was marked by the loss of the median finfolds and the appearance of black blotches on the caudal, subdorsal, and supra-anal regions. The commencement of squamation and the appearance of the rudiments of maxillary barbels distinguished the juvenile stage. The subadults measuring 4.6 ± 0.36 cm TL had finished squamation and completely lost the subdorsal and supra-anal blotches. We propose that the presence of subdorsal blotches is a distinctive ontogenetic and systematic feature of larval and juvenile forms of smiliogastrin barbs.

Murray cod and modern fish screens: influence of water velocity and screen design on the entrainment and impingement of larval and young-of-year fish at water offtakes

Context Entrainment and removal of fish from aquatic ecosystems can occur at water pump offtakes. Exclusion screens that reduce these impacts are recognised as an important conservation measure. Aims Evaluate the effectiveness of the Australian screen design guidelines in protecting larvae and young-of-year age class of a native fish species, Murray cod Maccullochella peelii. Methods Entrainment and impingement of postflexion larvae and young-of-year were assessed in a controlled laboratory environment. Tests were conducted under a range of approach velocities (AV) and impingement durations for two screen materials. Key results Fish screens reduced larval entrainment by ≤84%. Screens had no significant effect on reducing larval entrainment at AV ≥0.125 m s−1. Impingement of young-of-year was positively associated with AV and mortality increased with impingement duration, irrespective of screen type. Conclusions To protect early life-stage Murray cod, it is recommended that water pump offtakes be fitted with 2-mm vertical wedge-wire stainless steel screens and AV be limited to ≤0.1 m s−1. Implications This study represents the first assessment of the effectiveness of the Australian screen design guidelines in protecting larvae, providing knowledge to further refine specifications for screen design and support the recovery of native fish populations.

First description of post-flexion larvae of the West Atlantic trumpetfish, Aulostomus maculatus

Abstract The West Atlantic trumpetfish Aulostomus maculatus is a species of little commercial importance, but it is frequently used as a study organism in behavioural ecology, and it has been traded in the aquarium industry to some extent. The adult life stage is well described, however its early life history is nearly unknown. This paper provides the first description of post-flexion larvae of A. maculatus, including detailed illustrations, photographs, morphological data, and collection site data of specimens collected during a multipurpose research survey conducted within the Sargasso Sea Subtropical Convergence Zone. The collection site also implies a geographic range expansion, off the continental shelf, of the pelagic larvae stage. This paper hence advances the scientific knowledge about the early life stages, distribution and ecology of this species.

Development and application of a bioenergetics growth model for multiple early life stages of an ecologically important marine fish

Spatial and temporal variability in temperature and food availability are key drivers of growth of marine fishes. Growth during the early life stages (ELS's) is tightly coupled to survival, and in turn, can set year-class strength (i.e. annual recruitment) and overall stock productivity of populations and fished stocks. Ontogenetic changes in physiology, dietary preferences, and growth across ELS's can be accounted for within bioenergetics models, but existing models lack resolution within larval and early juvenile stages. We leveraged daily output from a coupled physical-biogeochemical model to force a highly resolved ontogenetic bioenergetics model parametrized for an ecologically important rockfish in the California Current System. Size-at-age predictions closely track empirical growth trajectories of the ELS's. Scenario testing revealed that growth performance is disproportionately driven by changes in temperature compared to food availability. We then expanded the model to incorporate spatial climatological differences in temperature and prey concentration and found that preflexion growth potential is maximized in areas of historical spawning, suggesting the timing and location of reproduction is an adaptive strategy that places larvae in habitat favorable for survival. Growth potential for late-stage larvae (postflexion) is greatest over a broad areal extent, implying that if a particle tracking algorithm was coupled to the bioenergetics model, a wide range of larval dispersal pathways would place postflexion larvae in habitat suitable for rapid growth. Finally, growth potential of pelagic juveniles is maximized over the continental shelf and shelf-break, aligning with high juvenile catch rates from a fisheries-independent survey. In summary, this study (i) serves as a proof of concept that a bioenergetics model with high ontogenetic resolution can reproduce life stage-specific growth trajectories even though the underlying physiology data for model parameterization is imperfect and (ii) can aid future studies aimed at understanding how ecosystem processes interact with ontogenetic growth and changes in year class strength of early life stages of marine fishes.

Nursery areas of Micromesistius poutassou and Sardina pilchardus unveil their reproductive strategies in the northwestern Mediterranean Sea

AbstractWinter conditions in the NW Mediterranean cause instability of the water column and non‐geostrophic dynamics, such as vertical mixing and convection are significant. These events involve nutrient supply to the photic zone that can sustain high productivity. In this study, we aim to investigate the role of winter hydrodynamics on the spawning strategies of Sardina pilchardus and Micromesistius poutassou. Data were obtained on two oceanographic cruises (February 2017 and 2018) off the Catalan coast. The occurrence of S. pilchardus eggs very close to the coast indicated a clear preference of the species for spawning in coastal areas. Preflexion and postflexion larvae exhibited a slightly wider distribution showing a clear association with the cold, less saline and more productive coastal waters. Preflexion larvae of M. poutassou were found on the upper slope and over the shelf, being offshore limited by the shelf/slope front present all along the slope. The front would act as a barrier preventing their dispersion towards the open sea. M. poutassou larvae in advanced developmental stages were located close to the coast in the productive shelf waters, with instabilities of the front contributing to larval transport from offshore waters to the coast. The vertical distribution of both species showed high variability, not only related with the daily cycle or developmental stage, but also with the vertical structure of the water column. Overall, the results provide some clues on how the spawning strategies of both species may evolve under future scenarios of higher winter‐stratification, because of the global warming.

Diet composition and feeding strategy of larvae and juveniles of green riffle goby, Stiphodon elegans in Cimaja Estuary, Indonesia

Green riffle goby, Stiphodon elegans, is an amphidromous goby living in Cimaja River, Sukabumi. Postflexion and juvenile stages of this species migrate from the sea to the river estuary for the recruitment process both in the day and nighttime. The presence of prey strongly influences the success of the recruitment process. The present study, therefore, aims to reveal the diet preferences and feeding strategies of postflexion larvae and juveniles of the green riffle goby during the recruitment process. Postflexion larvae and juveniles of S. elegans were collected day and nighttime using a lift net from December 2020 to May 2021 in the Cimaja River estuary. A total of 13,955 larvae and juveniles were captured. For diet analysis, 455 fish larvae and juveniles were analyzed, consisting of 47 larvae (8.9-11.8 BL) and 408 juveniles (11.9-21.8 BL). The diet composition of postflexion larvae comprised five genera from three different classes of benthic microalgae and zoobenthic. In comparison, fish juveniles’ prey composition comprised 15 genera from 5 different classes of benthic microalgae and zoobenthic. The postflexion larva tends to be more specialized than the juvenile in utilizing prey. There is no significant difference in the diet composition and feeding strategy of postflexion larvae and juveniles during the day and nighttime (p>0.05).

Could extremophilia be a life history strategy in late-stage larvae of Sparodon durbanensis (Family Sparidae)?

Microbialite pools are dynamic, heterogenous environments with extreme physico-chemical variability in terms of temporal temperature, oxygen and salinity fluctuations. Despite this habitat being hypothesised to be an ecological trap for larvae of marine estuary-dependent fishes, late-stage larvae and early juvenile White musselcracker (Sparodon durbanensis) ubiquitously occur in these pools. This study aimed to investigate the relationship between S. durbanensis density, developmental stage, total length and the physico-chemical environment using a multiple linear regression model analysis. Moreover, it aimed to further explore the potential nursery role of these extreme habitats. All larval and early juvenile stage S. durbanensis were collected with hand nets from pools at three representative microbialite sites on the warm temperate coast of South Africa from late summer to late autumn in 2021. A total of 63 pools were sampled with a total of 57 postflexion larvae and 162 early juvenile S. durbanensis specimens collected. Pool temperatures ranged from 16.6 to 37.3 °C with a mean of 24.5 °C, dissolved oxygen concentration ranged from 5.5 to 26.0 mg/L with a mean of 13.5 mg/L, and salinity ranged from 1.3 to 34.8 with a mean of 25.4. Temperature had a significant positive relationship with total length for early juvenile stages, while none of the other predictor variables showed evidence of a significant relationship with total length. Fish density showed a significant relationship with pool volume for both developmental stages where higher densities occurred in smaller pools that were also characterised by the most extreme physico-chemical variability. Late-stage larvae and early juveniles of this marine fish, that does not use estuaries as a nursery area, appears to complete the larval phase under extreme estuarine-like conditions in microbialite pools. Microbialite pools, as well as adjacent non-microbialite rocky shore pools appear to act as a micro-refuge, possibly improving growth rates and reducing the pelagic larval duration due to warmer water temperatures after which fish move into adjacent deeper, low shore pools and gullies. Due to the extreme conditions in microbialite pools, protection from predators during transition from larval to juvenile stages is also afforded.

Selective feeding of the mullet larvae Liza haematocheila during ontogeny in Laizhou Bay, Bohai Sea, China: The importance of small copepods in mesozooplankton as prey

The mullet Liza haematocheila is widely distributed in low-salinity waters around the world and has high economic value. However, details regarding the foraging ecology of mullet larvae remain unclear. Larvae of L. haematocheila were sampled in Laizhou Bay of the Bohai Sea, China, in May 2016, and diet composition was detected using gut content analysis to compare differences in feeding parameters and diet shift in dominant prey during ontogeny. The results showed no linear relationship between gape size (GS) and standard length (SL) at larva length <7 mm (flexion larvae, FL), but linear increase was observed for SL >7 mm (post-flexion larvae, PFL). Maximum prey size (MPS) overlapped with GS during the FL stage but was never higher than GS during the PFL stage. Trends of increasing MPS and prey size range (PSR) during the PFL stage were lower than those during the FL stage, but prey number (PN) increased significantly during the PFL stage. Diet composition analyses in mullet larvae showed a total of 10 mesozooplankton species (or categories), of which 8 species were copepods (including copepods nauplii and copepodites), and showed the dominance of 4 small copepods (<1 mm). Analyses of the numerical proportion of dominant copepods showed that the largest prey (Paracalanus parvus) gradually increased as GS increased; conversely, the smallest prey (nauplii of Calanus sinicus) decreased. Collectively, these results suggest that PFL tends to exhibit increased PN but not prey size or size range, and diet shifts from smaller to larger prey during ontogeny in mullet larvae. All these indicate that PFL has higher prey selection ability compared with FL, specifically switching the diet to include larger small copepods during the PFL stage and increasing the prey number instead of increasing prey size. These determine the importance of small copepods in mesozooplankton as dominant prey and facilitate predictions of the impact of climate change on mesozooplankton and fish larvae.

Allometric Growth Patterns and Ontogenetic Development during Early Larval Stages of Schizothorax waltoni Regan and Percocypris retrodorslis in Southwest China

Due to a sharp decline in resources, Schizothorax waltoni Regan and Percocypris retrodorslis have been listed as wildlife under second-class protection in China. Under culture conditions, the early development and allometric growth patterns of S. waltoni and P. retrodorslis were researched from the hatching stage to 60 DPHs (days post-hatching), and a sampling of ten to fifteen larvae was made every day, followed by measurements with Axio Vision 4.8 software (Carl Zeiss AG, Jena, Germany). Morphological indicators included the anal fin length, the body depth, the body depth at the anus level, the caudal fin length, the dorsal fin length, the eye diameter, the head length, the head depth, the pectoral fin length, the tail length, the trunk length, the snout length, the total length, and the ventral fin length. Based on the morphology development of S. waltoni Regan and P. retrodorslis, four periods of larval growth were identified: pre-flexion larvae at 0–14 DPHs and 0–16 DPHs; flexion larvae at 14–23 DPHs and 16–26 DPHs, post-flexion larvae at 23–50 DPHs and 26–52 DPHs, and the juvenile stage. In newly hatched larvae, most organs and body parts were not differentiated, and they successively developed within 26 DPHs. The depletion of the yolk sac was observed at 23 DPHs and 25 DPHs. Allometric growth mainly occurs in the head and tail regions, indicating that body parts related to feeding and swimming behaviors were more important than the other parts. In addition, the growth pattern shows that the development of organs gives priority to the functions of gill respiration, sensation, exogenous feeding, and swimming. The inflection points of body part growth patterns only appeared before 40 DAH, so future studies should concentrate focus on developing the best feeding from the first feeding to 40 DAH. These outcomes were discussed with regard to the ontogeny of the functional morphology in relation to ecology and aquaculture. It is expected that this research can provide valuable perspectives in species conservation.

Allometric growth and larval development in Pacific red snapper Lutjanus peru under culture conditions.

Allometric growth is a common feature during fish larval development. It has been proposed as a growth strategy to prioritize the development of body segments related to primordial functions like feeding and swimming to increase the probability of survival during this critical period. In the present study we evaluated the allometric growth patterns of body segments associated to swimming and feeding during the larval stages of Pacific red snapper Lutjanus peru. The larvae were kept under intensive culture conditions and sampled every day from hatching until day 33 after hatching. Each larva was classified according to its developmental stage into yolk-sac larva, preflexion larva, flexion larva or postflexion larva, measured and the allometric growth coefficient of different body segments was evaluated using the potential model. Based on the results we can infer the presence of different ontogenetic priorities during the first developmental stages associated with vital functions like swimming during the yolk-sac stage [total length (TL) interval=2.27-3.005 mm] and feeding during the preflexion stage (TL interval=3.007-5.60 mm) by promoting the accelerated growth of tail (post anal) and head, respectively. In the flexion stage (TL interval=5.61-7.62 mm) a change in growth coefficients of most body segments compared to the previous stage was detected, suggesting a shift in growth priorities. Finally, in the postflexion stage (TL interval=7.60-15.48 mm) a clear tendency to isometry in most body segments was observed, suggesting that growth priorities have been fulfilled and the larvae will initiate with the transformation into a juvenile. These results provide a framework of the larval growth of L. peru in culture conditions which can be useful for comparative studies with other species or in aquaculture to evaluate the changes in larval growth due to new conditions or feeding protocols.

Larval Development of Siamese Fighting Fish (Betta splendens Regan, 1910)

In this study, the larval development stages of one of the most important ornamental fish Siamese fighting fish betta (Betta splendens Regan, 1910) were examined. The larval development stage was documented from hatching until the beginning of the juvenile period. The fertilized eggs were incubated at a water temperature of 28±0.5°C. The embryonic developmental stage was completed and hatching was observed at 28-32 hours post fertilization (hpf). The mouth and anus were closed 1 day after hatching (DAH). The mouth and anus were opened at 2 DAH. Exogenous feeding started on 3 DAH. The swim bladder was inflated at 2–3 DAH. The yolk sac was completely consumed at 4 DAH and the larvae began to swim freely. The larval development of Betta splendens was divided into four different periods: Yolk-sac larva (1–4 DAH), preflexion larva (4–8 DAH), flexion larva (11-12 DAH) and post-flexion larva (13–30 DAH). The larval metamorphosis was completed, and the larvae transformed into juveniles at 30 DAH.

The nursery function of shallow nearshore and estuarine benthic habitats for demersal fishes

Estuaries and shallow marine nearshore areas are highly productive and valuable ecosystems, which provide numerous habitats for fish and support fundamental ecological links with other environments. Yet only a few studies have focussed on fish community patterns across an estuary and marine nearshore gradient concurrently to determine the nursery value of both environments. This research gap is particularly apparent in southern Africa. The main aim of this study was to assess the relative roles of two permanently open estuaries and the adjacent marine nearshore areas of Algoa Bay, on the south coast of temperate South Africa, as settlement and nursery areas for demersal fishes. Soft-bottom habitats in both the estuaries and the nearshore were dominated by early-life stages (postflexion larvae, settlement stage and young-of-the-year fishes), but a significantly greater density of early-life stage fishes was found in the estuarine environment, highlighting the importance of estuarine habitats for demersal fishes within the Algoa Bay nursery seascape. Settlement and nursery habitats also differed by estuarine-association, with the marine nearshore environment dominated by marine and marine estuarine-opportunists, with core settlement and nursery areas for these species located close to estuary mouths, freshwater outlets and harbours. In contrast, the estuarine environment was dominated by marine estuarine-dependent and estuarine and marine species. This highlights the importance of concurrently sampling habitats in marine nearshore and estuarine environments, which allows for a better understanding of the degree of estuarine dependency of marine fish as well as an understanding of the nursery function of coastal habitats.

Distribution, abundance, and species composition of fish larvae and juveniles of Gobiidae in the Cimaja estuary, Palabuhanratu, Indonesia

Cimaja estuary has been well known as a fishing ground for fish larvae and juveniles of amphidromous fishes. The local people call this group of fish impun and collect them frequently for consumption. However, no information about fish diversity and recruitment of amphidromous goby in the Cimaja estuary is known. Therefore, this study aims to reveal species composition, recruitment pattern, and estimation of the abundance of fish larval and juvenile amphidromous goby in the Cimaja estuary day and night. Sampling was performed monthly during the new moon period from December 2020 to February 2021. Fish specimens were identified to the lowest taxon level through a morphological approach. A total of 23,331 larvae and juveniles of Gobiidae were collected, and most of them were Sicyopterus spp. The juvenile stage (11.9-21.8 mm BL) comprised 95.6% of the catches, postflexion larvae (6.9-11.8 mm BL) contributed 3.5% to the total yield and 0.9% of the total capture consisting of flexion larvae (3.9-6.8 mm BL). Gobiid fishes are preferred to recruit and migrate into the Cimaja river during the day than at night time (p < 0.05), and their movement to the upstream are synchronized with the tidal cycle.

Temporal variation in abundance of bully sleepers (Pisces: Eleotridae) larvae and juveniles in Cisolok Estuary, Palabuhanratu Bay, Indonesia

Larvae and juveniles of bully sleepers are the primary targets for the local community surrounding Cisolok River, and this traditional fishing activity is called nyalawean. The present study aims to uncover fish larval diversity and recruitment of bully sleepers in the Cisolok river estuary. Fish sampling using a lift net was conveyed monthly from December 2020 to February 2021. Fish specimens were identified to the lowest taxon through a morphological approach and DNA Barcoding. Catch per unit effort measured the abundance of fish larvae and juveniles (CPUE, Ind.h−1). Only one species was found during the study period, namely Bunaka gyrinoides. A total of 7,353 larvae and juveniles of B. gyrinoides were captured, consisting of flexion (45 individuals; 4-5.9 mm BL), postflexion (6,887; 6-11 mm BL), and early juvenile (441; 11.1-14.8 mm BL). Postflexion larvae were the most abundant specimens, contributing 88.4-95.5%; 96-81.1%, 94.2-97.8% to the total catches in December, January, and February. Larvae and juveniles were more abundant during daylight (671-1591 Ind.h-1) rather than nighttime (147-567 Ind.h-1). These findings indicate that bully sleepers recruit and migrate upstream into the freshwater area of the Cisolok River during the day and nighttime synchronized with the tidal fluxes.

A productivity bottleneck in the Baltic herring (Clupea harengus membras): Early life-history processes and recruitment variability

Exogenous anomalies induced by contemporary climate change may severely impact dynamics of early life stages of fish. Here, we modelled how growth rate and abundance of postflexion larvae, and recruitment of Baltic spring-spawning herring (Clupea harengus membras) in the Pärnu Bay, Gulf of Riga (GoR) may respond to shifting climate variables. Higher larval growth rates were aligned with later seasonal emergence of yolk-sac larvae, while lower abundance of postflexion larvae occurred in years of earlier seasonal seawater warming. Cooler temperatures (<16 °C) in spring expanded the optimal thermal window for first-feeding herring larvae, attributable to the absence of early seasonal water temperature warming. Higher recruitment levels emerged in years of seasonally delayed warming and were associated with higher abundance of postflexion larvae. In recent decades, the trend towards earlier warming of the Baltic Sea in spring threatens to create a bottleneck to successful recruitment of herring. The existing paradigm that abundant Baltic herring year-classes occur only in the years following mild winters no longer stands as environmental conditions undergo rapid change. The relative contribution of Pärnu Bay larval nursery areas to recruitment has diminished as the suitable thermal window has been dramatically reduced in recent decades. Evolving thermal dynamics in the GoR have developed relatively recently and in future present a bottleneck for herring production.

First faunal record of larval and juvenile gobies (Actinopterygii: Gobiiformes) in the Ka Long estuary, northern Vietnam

Gobies are dominant in terms of both species diversity and abundance in estuarine habitats, but little is known about their early developmental stages in the coastal waters of Vietnam. To examine species diversity of goby larvae and juveniles, monthly collections were carried out using a small seine net from the Ka Long estuary in the northernmost Vietnamese coast, from September 2014 to August 2015. A total of 1,334 goby fishes of 20 species or more in three families (Butidae, Oxudercidae and Gobiidae) were caught, including nine unidentified species. This is the first record of gobies at early developmental stages from an estuary in Vietnam. The family Oxudercidae was the most abundant in terms of total species, and Gobiopterus chuno was the most frequent species in the study area. The goby specimens collected were mostly postflexion larvae and juveniles, ranging from 2.3 mm to 99.1 mm in body length. The size of nine dominant gobies was around 10 mm body length. In the study site, goby fishes are from tropical, subtropical and temperate zones, suggesting this goby fauna can display transitional biodiversity from tropical to temperate waters.

Association of early stage fishes with surf diatom Anaulus australis accumulations in a warm temperate surf zone, South Africa

Surf zones are recognised as important nursery habitats for early stage fishes worldwide, but microhabitat opportunities for feeding and refuge are not well studied. Surf diatoms accumulate in high densities, discolouring the water and offering potential refuge and feeding opportunities. The aim of this study was to investigate the association of early stage fishes with surf diatom Anaulus australis accumulations in the Sundays Beach surf zone on the South African southeast coast. Early stage fishes were sampled within and outside of visible A. australis accumulations using a 4.5 × 1.5 m larval fish seine net with 500 μm mesh. Over 22 sampling trips, 338 replicate samples were collected. Species diversity, the overall catch per unit effort (CPUE) and that of the dominant species Chelon richardsonii, Diplodus capensis, Rhabdosargus globiceps and R. holubi, were significantly higher within accumulations compared to outside. Postflexion larvae and early juveniles occurred in significantly higher numbers within accumulations, indicating that fishes were actively selecting A. australis accumulations. Generalized linear models revealed that temperature, dissolved oxygen (mg l−1), turbidity and A. australis biomass (μg Chl a l−1) were the most significant variables correlated with the overall CPUE and that of dominant species. Since A. australis accumulations have a patchy distribution in the surf zone, it is suggested that early stage fishes are opportunistically using this resource when and where it occurs as both a feeding and refuge area. Understanding the role of microhabitats for early stage fishes is essential for understanding survival and life history strategies within the mosaic of coastal nursery areas.

Ontogeny versus environmental forcing off the Southwest Atlantic Ocean: Nutritional condition of Fuegian sprat (Sprattus fuegensis) early stages

AbstractThe Fuegian sprat, Sprattus fuegensis, plays a key trophic role in the Southwest Atlantic Ocean. Growth and survival of small pelagic fishes like sprat are strongly affected by environmental variability and can determine recruitment success. We estimated the nucleic acid composition and assessed with a standardized RNA/DNA index (sRD) the nutritional condition of 273 larvae, metamorphosing, and juvenile Fuegian sprat from two environmentally distinct habitats: the waters off Tierra del Fuego (TDF) and the Marine Protected Area Namuncurá/Burdwood Bank (MPAN/BB). A similar ontogenetic pattern, in which nucleic acid concentrations decreased and sRD increased with increasing standard length (SL) among larval stages was observed in both habitats. A higher percentage of preflexion larvae were under the sRD threshold for growth and optimal growth performance, with sRD values being significantly higher in TDF. Postflexion larvae in both habitats showed maximum sRD values, suggesting that both habitats are suitable nursery grounds. In contrast, metamorphosing and juvenile sprat (only captured in TDF) had consistently low nucleic acid concentrations and sRD values despite increasing SL. Environmental forcing (temperature, salinity, and station depth) was assessed over size‐corrected larval sRD. The best model included a negative response to station depth, and the effect of processes associated with this factor are discussed. Although further analyses are needed to reveal underlying dynamics determining early development, these results comprise a baseline for future monitoring approaches on Fuegian sprat life traits and factors affecting their recruitment in this region.

Plankton food webs in the oligotrophic Gulf of Mexico spawning grounds of Atlantic bluefin tuna.

We used linear inverse ecosystem modeling techniques to assimilate data from extensive Lagrangian field experiments into a mass-balance constrained food web for the Gulf of Mexico open-ocean ecosystem. This region is highly oligotrophic, yet Atlantic bluefin tuna (ABT) travel long distances from feeding grounds in the North Atlantic to spawn there. Our results show extensive nutrient regeneration fueling primary productivity (mostly by cyanobacteria and other picophytoplankton) in the upper euphotic zone. The food web is dominated by the microbial loop (>70% of net primary productivity is respired by heterotrophic bacteria and protists that feed on them). By contrast, herbivorous food web pathways from phytoplankton to metazoan zooplankton process <10% of the net primary production in the mixed layer. Nevertheless, ABT larvae feed preferentially on podonid cladocerans and other suspension-feeding zooplankton, which in turn derive much of their nutrition from nano- and micro-phytoplankton (mixotrophic flagellates, and to a lesser extent, diatoms). This allows ABT larvae to maintain a comparatively low trophic level (~4.2 for preflexion and postflexion larvae), which increases trophic transfer from phytoplankton to larval fish.