Seasonal occupancy and connectivity amongst nearshore flats and reef habitats by permit Trachinotus falcatus: considerations for fisheries management

We used acoustic telemetry to quantify permit Trachinotus falcatus habitat use and connectivity in proximity to the Florida Keys, USA, and assessed these patterns relative to current habitat and fisheries management practices. From March 2017 to June 2018, 45 permit tagged within 16 km of the lower Florida Keys were detected at stationary acoustic receivers throughout the south Florida region, the majority of which remained within the Special Permit Zone, where more extensive fisheries harvest regulations are implemented. There was a high level of connectivity between nearshore flats (i.e., <3 m water depth) and the Florida reef tract (FRT; 15–40 m water depth), with 75% of individuals detected in both habitats. These locations probably function primarily as foraging and spawning habitats, respectively. Permit occupancy on the FRT peaked during the months of March–September, with the highest number of individuals occurring there in April and May. Specific sites on the FRT were identified as potentially important spawning locations, as they attracted a high proportion of individuals that exhibited frequent visits with high residency durations. There were also significant positive relationships between seasonal habitat-use metrics on the FRT and an empirical permit gonadosomatic index. Large aggregations of permit at spawning sites on the FRT are potentially vulnerable to the effects of fishing (including predation during catch and release) at a critical point in their life cycle. These data on permit space use and movement, coupled with knowledge of stressors on their ecology, provide insights for implementing science-based strategic management plans.     

Early life history and settlement of the slender filefish, <Emphasis Type="Italic">Monacanthus tuckeri</Emphasis> (Monacanthidae), at Calabash Caye,Turneffe Atoll,Belize

Synopsis We examined early life history traits and patterns of settlement of the slender filefish, Monacanthus tuckeri, at Calabash Caye, Turneffe Atoll, Belize. A settlement peak was evident at the new moon, and no settlement occurred at the full moon. However, settlement rates at the quarter moons could not be estimated due to sampling gaps. Many reef fishes show new moon settlement peaks, so M. tuckeri shares some characteristics with the primarily perciform species on coral reefs. Pelagic larval duration was long (mean = 42 days) and variable, suggesting that dispersal patterns might be diverse. Size at settlement was large (mean = 32 mm total length) and also variable. Larval duration and size at settlement were outside of the average values exhibited by reef fishes, but are not beyond the extreme end of the range, and might be explained by association with pelagic debris prior to settlement. There were no differences in overall settlement rates on reef and seagrass habitats, and fish settling to either habitat did not differ in larval duration, size at settlement, or larval growth rate. This suggests that settlement to alternative habitats may be random, or driven by availability of suitable microhabitat, rather than habitat quality or individual traits.     

The effects of exposure in sandy beach surf zones on larval fishes

The influence of wind and wave exposure on larval fish assemblages within a large bay system was investigated. Larval fishes were sampled from two areas with vastly different exposure to waves and wind, namely the windward and leeward sectors of Algoa Bay. In total, 5702 larval fishes were collected using a modified larval seine. Of these, 4391 were collected in the leeward and 1311 in the windward sector of the bay, representing a total of 23 families and 57 species. Dominant fish families included Clinidae, Engraulidae, Kyphosidae, Mugilidae, Soleidae and Sparidae, similar to the situation elsewhere, highlighting continuity in the composition of larval fish assemblages and the utilization of surf zones by a specific group of larval fishes. Nineteen estuary‐associated marine species occurred within the surf zones of Algoa Bay and dominated catches (86·7%) in terms of abundance. Postflexion larvae comprised > 80% of the catch, indicating the importance of the seemingly inhospitable surf zone environment for the early life stages of many fish species. The greatest species diversity was observed within the windward sector of the bay. Distance‐based linear modelling identified wave period as the environmental variable explaining the largest proportion of the significant variation in the larval fish assemblage. The physical disturbance generated by breaking waves could create a suitable environment for fish larvae, sheltered from predators and with an abundance of food resources.     

The relative importance of habitat-specific settlement, predation and juvenile dispersal for distribution and abundance of young juvenile shore crabs Carcinus maenas L.

Young juveniles of many motile benthic species are concentrated in structurally complex habitats, but the proximate causes of this distribution are usually not clear. In the present study, I assessed three potentially important processes affecting distribution and abundance of early benthic stages in the shore crab (Carcinus maenas): (1) selection of habitat by megalopae (postlarvae); (2) habitat-specific predation; and (3) post-settlement movements by juveniles. These processes were assessed concurrently over 3-9 days at two spatial scales: at the scale of square meters using cage techniques within nursery areas, and at the scale of hectares using isolated populations of juvenile shore crabs in small nursery areas as mesocosms. The results were compared to habitat-specific distribution in the field.Shore crab megalopae and first instar juveniles (settlers) were distributed non-randomly among micro-habitats in the assessed nursery areas, with great densities in both mussel beds, eelgrass and filamentous algal patches (on average 114-232 settlers m⁻²), and significantly smaller densities on open sand habitats at all times (on average 4 settlers m⁻²). The same habitat-specific settlement pattern was found in cages where predators were excluded, suggesting that active habitat selection at settlement was responsible for the initial distribution. Older juveniles (second to ninth instar crabs) were also sparse on sand, but in contrast to settlers, were concentrated in mussel beds, which showed significantly greater densities than eelgrass and algal habitats. The cage experiment demonstrated a dynamic distribution of juvenile crabs. Young juveniles constantly migrated over open sand habitats (20 m or further) and colonized the experimental plots in a habitat-specific pattern that reflected the distribution in the field. This pattern was also found for very small crabs colonizing predator-exclusion cages, suggesting that selection of habitat by migrating juveniles caused the ontogenetic change in habitat use. Although post-settlement movements were great within nursery areas, juvenile dispersal at a regional scale appeared to be small, and the recruitment of juvenile shore crabs to the shallow bays occurred mainly through pelagic megalopae.Conservative estimates at the scale of whole nursery areas, based on migration trap data and field samples, indicated great mortality of settlers and early benthic stages of shore crabs. Results from the cage experiment suggest that predation by crabs and shrimp were responsible for the high settlement mortality. Both enclosed cannibalistic juvenile crabs and local predators on uncaged habitat plots caused significant losses of settlers in all habitats (on average 22% and 64% 3 day⁻¹, respectively). The effect of predators was highly variable between trials, but differed little between habitat types, and predation had no detectable proximate effect on juvenile distribution, despite the great losses. Small settlement densities on sand habitats in combination with a refuge at low prey numbers, and an aggregation of cannibalistic juvenile crabs in nursery habitats appear to decrease the effect of habitat-specific predation rates on the distribution of juvenile shore crabs. This study demonstrates that active habitat selection at settlement followed by a dynamic redistribution of young juveniles can be the proximate processes responsible for habitat-specific distribution of epibenthic juveniles, and indicate that predation represents a major evolutionary process reinforcing this behavior.     

A review on the early life history and ecology of Japanese sea bass and implication for recruitment

Recruitment in marine fishes is regulated largely by the demographic changes that occur during the early life stages; therefore, a thorough understanding of early life stages is essential for predicting recruitment variability in fishes. Japanese sea bass (JSB), Lateolabrax japonicus, is a coastal marine fish distributed in East Asian coastal waters, and is regarded as highly important for commercial and recreational fisheries, for marine and brackish water aquaculture as well as for stock enhancement. JSB is a typical estuarine dependent temperate fish, which spawns in shelf areas and coastal embayments and the larvae and juveniles are dispersed and transported into shallow nearshore habitats and estuaries where they spend the early life. In this paper, we provide insight into the early life history and ecology of JSB through a revision of the available information and using the data we obtained from a relatively long-term research. We review and discuss the distribution and habitat use, food and feeding, age and growth, mortality and recruitment of larval and juvenile JSB in coastal waters around Japan. We extend our discussions in all available dimensions: habitat-specific, ontogenetic, and spatio-temporal, and highlight the importance of nursery habitats. We also discuss the implications of early life history for recruitment of JSB as well as the possible effects of climate change. At the end, we point out potential areas for future research.     

Stable isotope analysis of vertebrae reveals ontogenetic changes in habitat in an endothermic pelagic shark

Ontogenetic changes in habitat are driven by shifting life-history requirements and play an important role in population dynamics. However, large portions of the life history of many pelagic species are still poorly understood or unknown. We used a novel combination of stable isotope analysis of vertebral annuli, Bayesian mixing models, isoscapes and electronic tag data to reconstruct ontogenetic patterns of habitat and resource use in a pelagic apex predator, the salmon shark (Lamna ditropis). Results identified the North Pacific Transition Zone as the major nursery area for salmon sharks and revealed an ontogenetic shift around the age of maturity from oceanic to increased use of neritic habitats. The nursery habitat may reflect trade-offs between prey availability, predation pressure and thermal constraints on juvenile endothermic sharks. The ontogenetic shift in habitat coincided with a reduction of isotopic niche, possibly reflecting specialization upon particular prey or habitats. Using tagging data to inform Bayesian isotopic mixing models revealed that adult sharks primarily use neritic habitats of Alaska yet receive a trophic subsidy from oceanic habitats. Integrating the multiple methods used here provides a powerful approach to retrospectively study the ecology and life history of migratory species throughout their ontogeny.     

The Great Melting Pot. Common Sole Population Connectivity Assessed by Otolith and Water Fingerprints

Quantifying the scale and importance of individual dispersion between populations and life stages is a key challenge in marine ecology. The common sole (Solea solea), an important commercial flatfish in the North Sea, Atlantic Ocean and the Mediterranean Sea, has a marine pelagic larval stage, a benthic juvenile stage in coastal nurseries (lagoons, estuaries or shallow marine areas) and a benthic adult stage in deeper marine waters on the continental shelf. To date, the ecological connectivity among these life stages has been little assessed in the Mediterranean. Here, such an assessment is provided for the first time for the Gulf of Lions, NW Mediterranean, based on a dataset on otolith microchemistry and stable isotopic composition as indicators of the water masses inhabited by individual fish. Specifically, otolith Ba/Ca and Sr/Ca profiles, and δ¹³C and δ¹⁸O values of adults collected in four areas of the Gulf of Lions were compared with those of young-of-the-year collected in different coastal nurseries. Results showed that a high proportion of adults (>46%) were influenced by river inputs during their larval stage. Furthermore Sr/Ca ratios and the otolith length at one year of age revealed that most adults (∼70%) spent their juvenile stage in nurseries with high salinity, whereas the remainder used brackish environments. In total, data were consistent with the use of six nursery types, three with high salinity (marine areas and two types of highly saline lagoons) and three brackish (coastal areas near river mouths, and two types of brackish environments), all of which contributed to the replenishment of adult populations. These finding implicated panmixia in sole population in the Gulf of Lions and claimed for a habitat integrated management of fisheries.     

Linking larval history to juvenile demography in the bicolor damselfish Stegastes partitus (Perciformes: Pomacentridae)

Otolith-based reconstructions of daily larval growth increments were used to examine the effect of variation in larval growth on size and age at settlement and post-settlement growth, survival and habitat preferences of juvenile bicolor damselfish (Stegastes partitus Poey). During August 1992 and 1994, newly settled S. partitus were collected from Montastraea coral heads and Porites rubble piles in Tague Bay, St. Croix, U.S. Virgin Islands (17 degrees 45'N, 64 degres 42' W). Daily lapillar otolith increments from each fish were counted and measured with Optimas image analysis software. S. partitus pelagic larval duration was 23.7 d in 1992 (n = 70) and 24.6 d in 1994 (n = 38) and larval age at settlement averaged 13.0 mm total length both years. Analysis of daily otolith increments demonstrated that variation in larval growth rates and size and age at settlement had no detectable effect on post-settlement survivorship but that larger larvae showed a preference for Montastraea coral at settlement. Late larval and early juvenile growth rates showed a significant positive relationship indicating that growth patterns established during the planktonic stage can span metamorphosis and continue into the benthic juvenile phase. Larval growth rates during the first two weeks post-hatching also had a strong effect on age to developmental competence (ability to undergo metamorphosis) in both 1992 and 1994 with the fastest growing larvae being 8 d younger and 0.8 mm smaller at settlement than the slowest growing larvae. These differential growth rates in early stage larvae established trajectories toward larval developmental competence and may prove important in biogeographical studies of larval dispersal.     

Marine reserve design theory for species with ontogenetic migration

Models for marine reserve design have been developed primarily with ‘reef fish’ life histories in mind: sedentary adults in patches connected by larval dispersal. However, many fished species undertake ontogenetic migrations, such as from nursery grounds to adult spawning habitats, and current theory does not fully address the range of reserve options posed by that situation. I modelled a generic species with ontogenetic migration to investigate the possible benefits of reserves under three alternative scenarios. First, the fishery targets adult habitat, and reserves can sustain yields under high exploitation, unless habitat patches are well connected. Second, the fishery targets the nursery, and reserves are highly effective, regardless of connectivity patterns. Third, the fishery targets both habitats, and reserves only succeed if paired on adjacent, well-connected nursery and adult patches. In all cases, reserves can buffer populations against overexploitation but would not enhance fishery yield beyond that achievable by management without reserves. These results summarize the general situations in which management using reserves could be useful for ontogenetically migrating species, and the type of connectivity data needed to inform reserve design.     

Multiple cues from multiple habitats: Effect on metamorphosis of the Florida stone crab, Menippe mercenaria

The Florida stone crab, Menippe mercenaria, is an economically and ecologically important species that ranges from North Carolina throughout the Caribbean and the southeastern Gulf of Mexico. However, there is little known about its early life history stages as compared to other commercially important species in the region. The goal of this research was to examine effects of putative cues on metamorphosis from the megalopa stage to the first juvenile stage. Our study investigated the effect of water-soluble exudates from four substrata, as well as natural biofilms, and exudates from adult stone crabs. In addition, the influence of natural substrata was compared to that of artificial substrata. Adult exudate had no significant effect on metamorphosis, despite a wide range of tested concentrations. In contrast, there was a significant effect on mean time to metamorphosis in experimental groups exposed to multiple cues associated with the brown alga Sargassum fluitans, rubble from stone crab habitat, the eastern oyster Crassostrea virginica, and biofilms associated with the oyster. Furthermore, we provide evidence for metamorphic responses to water-soluble chemical cues, as well as biochemical and physical cues associated with different substrata. Overall results were coherent with the relevant body of previous work on metamorphosis of brachyuran crab larvae and indicate that both physical and chemical cues are important factors in facilitating the settlement and metamorphosis of M. mercenaria larvae in juvenile nursery habitat.     

Influence of Biological Factors on Connectivity Patterns for Concholepas concholepas (loco) in Chile

In marine benthic ecosystems, larval connectivity is a major process influencing the maintenance and distribution of invertebrate populations. Larval connectivity is a complex process to study as it is determined by several interacting factors. Here we use an individual-based, biophysical model, to disentangle the effects of such factors, namely larval vertical migration, larval growth, larval mortality, adults fecundity, and habitat availability, for the marine gastropod Concholepas concholepas (loco) in Chile. Lower transport success and higher dispersal distances are observed including larval vertical migration in the model. We find an overall decrease in larval transport success to settlement areas from northern to southern Chile. This spatial gradient results from the combination of current direction and intensity, seawater temperature, and available habitat. From our simulated connectivity patterns we then identify subpopulations of loco along the Chilean coast, which could serve as a basis for spatial management of this resource in the future.     

Variability in water temperature affects trait-mediated survival of a newly settled coral reef fish

As animals with complex life cycles metamorphose from one stage to the next, carry-over effects from earlier stages can affect future mortality. To examine the relationship between early life history traits and survival, seven monthly cohorts of newly-settled bluehead wrasse Thalassoma bifasciatum were collected immediately after settlement and over sequential 3-day periods. Otolith analysis was used to quantify mean larval and juvenile growth rates, pelagic larval duration (PLD), and settlement size and condition of different age classes to identify the traits most important for survival. Overall, survivors tended to have shorter PLDs, to settle at smaller sizes and higher condition levels, and to exhibit faster early juvenile growth. Water temperature contributed to among-cohort variability in traits as warmer water led to faster larval and juvenile growth and shorter PLDs. Trait-specific fitness functions demonstrated that temperature can influence fitness by changing the nature of selection on each trait. Estimates of selection intensity revealed that settlement condition contributed the most to variation in fitness across cohorts, followed by juvenile growth. Frequent loss of low settlement condition individuals and occasional loss of the very highest condition fish suggest that particularly high settlement condition during the warmest temperatures may be detrimental. Not only does the quality of settlers vary over time, but selective loss of individuals with particular phenotypic traits is not pervasive and can vary with environmental conditions such as temperature.     

Do surf zones in New Jersey provide “nursery” habitat for southern fishes?

The fishes of the energetic surf zone of exposed ocean beaches are poorly known, especially along the U.S. east coast. We investigated the role that surf zones in southern New Jersey have as habitat by describing the young-of-the-year fishes caught by seines within the surf zone from May through October in 1998, 1999, 2005 and 2006. In addition, we investigated the nursery role of these surf zones by examining abundance, growth, and inferred survival during the summer for the dominant locally (New Jersey) and southern (south of Cape Hatteras) spawned species. At least three southern species, i.e. bluefish Pomatomus saltatrix (Cohort I), white mullet Mugil curema, and Florida pompano Trachinotus carolinus successfully use New Jersey surf zones as nurseries during the summer. It is clear that bluefish (Cohort I) contributes to the adult population and it is likely for the other southern species. Ultimately, the nursery contribution of surf zones and other habitats at the northern limits of the range of southern species depends on whether surviving juveniles can successfully emigrate to overwinter habitats and complete their life history.     

Temperature influences selective mortality during the early life stages of a coral reef fish

For organisms with complex life cycles, processes occurring at the interface between life stages can disproportionately impact survival and population dynamics. Temperature is an important factor influencing growth in poikilotherms, and growth-related processes are frequently correlated with survival. We examined the influence of water temperature on growth-related early life history traits (ELHTs) and differential mortality during the transition from larval to early juvenile stage in sixteen monthly cohorts of bicolor damselfish Stegastes partitus, sampled on reefs of the upper Florida Keys, USA over 6 years. Otolith analysis of settlers and juveniles coupled with environmental data revealed that mean near-reef water temperature explained a significant proportion of variation in pelagic larval duration (PLD), early larval growth, size-at-settlement, and growth during early juvenile life. Among all cohorts, surviving juveniles were consistently larger at settlement, but grew more slowly during the first 6 d post-settlement. For the other ELHTs, selective mortality varied seasonally: during winter and spring months, survivors exhibited faster larval growth and shorter PLDs, whereas during warmer summer months, selection on PLD reversed and selection on larval growth became non-linear. Our results demonstrate that temperature not only shapes growth-related traits, but can also influence the direction and intensity of selective mortality.     

Genetic connectivity in the Florida reef system: comparative phylogeography of commensal invertebrates with contrasting reproductive strategies

Effective spatial management of coral reefs including design of marine protected areas requires an understanding of interpopulation genetic connectivity. We assessed gene flow along 355 km of the Florida reef system and between Florida and Belize in three commensal invertebrates occupying the same host sponge (Callyspongia vaginalis) but displaying contrasting reproductive dispersal strategies: the broadcast-spawning brittle star Ophiothrix lineata and two brooding amphipods Leucothoe kensleyi and Leucothoe ashleyae. Multiple analytical approaches to sequence variation in the mitochondrial COI gene demonstrated a high degree of overall connectivity for all three species along the Florida reef system. Ophiothrix lineata showed significant genetic structuring between Florida and Belize, and a pattern of isolation by distance but no significant genetic structuring along the Florida coastline. Bayesian estimates of migration detected a strong southerly dispersal bias for O. lineata along the Florida reef system, contrary to the general assumption of northerly gene flow in this region based on the direction of the Florida Current. Both amphipods, despite direct development, also showed high gene flow along the Florida reef system. Multiple inferences of long-distance dispersal from a nested clade analysis support the hypothesis that amphipod transport, possibly in detached sponge fragments, could generate the high levels of overall gene flow observed. However, this transport mechanism appears much less effective across deep water as connectivity between Florida and Belize (1072 km) is highly restricted.     

Connectivity between estuaries and marine environment: Integrating metrics to assess estuarine nursery function

Natural or anthropogenic induced variations in estuaries and the dynamics of marine fish populations potentially promote differences in connectivity between estuaries and marine areas, i.e. in their importance as nursery grounds. Within this context, an integrated assessment of the differential nursery function of the main estuaries along the Portuguese coast for commercial fish species common sole Solea solea, Senegalese sole Solea senegalensis, flounder Platichthys flesus and sea bass Dicentrarchus labrax was performed through several indicators based on available data. Contribution of individual estuaries to marine subpopulations was measured with potential metrics (juvenile density, habitat quantity, juvenile number and habitat quality within estuaries) and effective metrics (estuarine source of young adults in marine environment measured via otolith elemental fingerprints). The relationship between the two types of metrics was also assessed. Estuaries identified as important nursery and/or effective juvenile habitat (EJH) differed with species and no single estuary was best for all, highlighting species-specific regulation of nursery function. Multiple species assessment of nursery and EJH function differed among estuaries. Management and conservation of estuaries should focus on sites with higher contributions to adult subpopulations of multiple species. The importance of defining precise scientific and management objectives was emphasized by the different rankings of estuaries obtained with nursery or EJH criteria. Potential and effective contribution of estuaries were not significantly correlated, but in a quantitative analysis juvenile densities and number of juveniles seem related with effective contribution in some species. An agreement between potential and effective contributions of estuaries is concurrent with the acknowledged minor role of juvenile stage processes in regulation of recruitment to adult subpopulations.     

Early ontogenetic ecology of sofie Chondrostoma toxostoma: an integrated approach to organism‐environment relationships

The influence of early life development on the swimming performance of the endangered sofie Chondrostoma toxostoma was examined to highlight trends in organism‐environment relationships. Sudden occurrences of change in integrated function were found and these were most decisive, in particular with respect to microhabitat use, between the larval and juvenile periods of development. Stabilization of relative growth, i.e . end of the remodelling process (metamorphosis), occurred well after all larval characteristics (remnants of finfold and rapid allometric growth) had disappeared and all juvenile structures had appeared (nasal septa and complete scale cover). The fact that stabilization of relative growth coincided with dramatic shifts in microhabitat use (organism needs) as well as in swimming capacity (organism skills) suggests a more 'decisive' type of change in organism‐to‐environment interaction than one purely of form, i.e . shift from nursery to adult habitat.     

Thresholds in seascape connectivity: the spatial arrangement of nursery habitats structure fish communities on nearby reefs

Ecosystems are linked by the movement of organisms across habitat boundaries and the arrangement of habitat patches can affect species abundance and composition. In tropical seascapes many coral reef fishes settle in adjacent habitats and undergo ontogenetic habitat shifts to coral reefs as they grow. Few studies have attempted to measure at what distances from nursery habitats these fish migrations (connectivity) cease to exist and how the abundance, biomass and proportion of nursery species change on coral reefs along distance gradients away from nursery areas. The present study examines seascape spatial arrangement, including distances between habitats, and its consequences on connectivity within a tropical seascape in Mozambique using a seascape ecology approach. Fish and habitat surveys were undertaken in 2016/2017 and a thematic habitat map was created in ArcGIS, where cover and distances between habitat patches were calculated. Distance to mangroves and seagrasses were significant predictors for abundance and biomass of most nursery species. The proportions of nursery species were highest in the south of the archipelago, where mangroves were present and decreased with distance to nurseries (mangroves and seagrasses). Some nursery species were absent on reef sites farthest from nursery habitats, at 80 km from mangroves and at 12 km from seagrass habitats. The proportion of nursery/non-nursery snapper and parrotfish species, as well as abundance and biomass of seagrass nursery species abruptly declined at 8 km from seagrass habitats, indicating a threshold distance at which migrations may cease. Additionally, reefs isolated by large stretches of sand and deep water had very low abundances of several nursery species despite being within moderate distances from nursery habitats. This highlights the importance of considering the matrix (sand and deep water) as barriers for fish migration.     

Environmental variability, early life-history traits, and survival of new coral reef fish recruits

For benthic marine organisms with complex life cycles, conditionsexperienced by pelagic larvae can influence juvenile survival.Trait-specific selective mortality has been documented in thelaboratory and field, yet our knowledge of the factors contributingto the existence, strength, and consistency of natural selectivemortality is limited. We compiled previously published and unpublisheddata on the common Caribbean coral reef fish, Thalassoma bifasciatum,recruiting to Barbados, West Indies, and the upper Florida Keysto examine how environmental variability during pelagic larvallife influences the distribution of early life-history traitsexhibited by new recruits. We explored how the scope of variabilityin otolith-derived traits such as larval growth, pelagic larvalduration (PLD), size and condition at settlement, and earlyjuvenile growth influences the degree to which mortality ofjuveniles is selective. At both locations, contrasting oceanographicconditions (periodic passage of large low-salinity North BrazilCurrent [NBC] rings near Barbados and seasonal variation inwater temperature at Florida) led to significant differencesin the early life-history traits of recruits. Mortality wasmost frequently selective for the two most variable traits,condition at settlement and early juvenile growth. Environmentalvariability (including variation in predation pressure and stress-inducingconditions) also likely influences juvenile mortality and consequentlythe degree to which selective loss of particular traits occurs.As we begin to better understand the spatial, temporal, andspecies-specific circumstances in which events occurring duringlarval life influence juvenile performance, studies must alsobe extended to examine how these processes are translated intoadult fitness.     

Habitat and resource use by larval and juvenile fishes in an arid-land river (Rio Grande, New Mexico)

1. Studies of mesic temperate and tropical rivers suggest an important role for floodplain habitats as nursery areas for larval and juvenile fishes. In arid‐land rivers the extent and duration of flooding is diminished and habitats and resources used by larval fishes are poorly known. Our study documented habitat and resource use of larval fishes in the Rio Grande, New Mexico, an arid‐land river. 2. Spatial and temporal distribution of larval and juvenile fishes and their inferred microhabitat preferences were studied during spring, summer and autumn, 2003. Stable carbon (¹³C : ¹²C) and nitrogen (¹⁵N : ¹⁴N) isotope ratios were measured to identify nutrient sources and characterise trophic positions of young‐of‐year fishes in this system. 3. Some fishes recruited during high flows (in spring), whereas others recruited during low‐flow periods in late summer. Regardless of the timing of reproduction, microhabitats with lower current velocity and higher temperature appeared to serve as vital nursery grounds for Rio Grande fishes. Ephemeral backwaters and disconnected side channels held the highest abundance and diversity of larvae and juveniles. 4. Stable isotope analyses revealed that fish larvae obtained carbon predominately from algal production in early summer, but used organic carbon derived from emergent macrophytes as river discharge decreased in mid‐summer. This shift may have been facilitated by microinvertebrate prey that grazed down edible algae and then switched to macrophytes in mid‐summer. Nitrogen isotope ratios did not differ among species or early life stages, suggesting that larval and juvenile fishes use similar food resources, especially when restricted to isolated pools in summer.