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Variation in pelagic larval growth of Atlantic billfishes: The role of prey composition and selective mortality
Abstract and Figures
Atlantic blue marlin (Makaira nigricans) and sailfish (Istiophorus platypterus) larvae were collected from 10monthly cruises (June–October 2003 and 2004) across the Straits of Florida to test (1) whether
growth differed between the more productive western region near the Florida shelf, and the less productive eastern region
toward the Bahamas, and (2) whether growth was related to prey consumption. Examination of larval sagittal otoliths revealed
that instantaneous growth and daily growth during the first 2–3weeks of life did not vary significantly between the two regions
for either species. However, recent growth during the last two full days prior to collection was greater in the west for blue
marlin larvae. Recent growth of blue marlin larvae<9mm SL (primarily zooplanktivorous) was significantly related to prey
composition (faster growth when higher proportions of Farranula copepods were consumed). Western larvae grew faster and had higher proportions of Farranula in their guts. Trends for sailfish larvae were not significant. In both species, comparison of early growth between<9 and≥9mm
SL size groups indicated that growth trajectories diverged around 5–8mm SL, the time when billfish larvae become capable
of piscivory. Significantly faster growth of larger (older) larvae suggests that mortality was selective for fast growers
and that the transition to piscivory may be a critical point in the early life of billfish.
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... Beyond total carbon ingested, a proxy for energy intake, feeding success is also modulated by the capacity of ingested prey taxa to provide the suite of essential nutrients, such as amino acids and fatty acids, that are necessary for proper larval development (Rønnestad et al., 1999;Sargent et al., 1999). The larval prey field is composed of various taxa characterized by differences in potential to fulfill requirements relative to energy and essential nutrient intake, which likely explains why several larval fish species have been shown to selectively consume prey taxa that result in their optimal growth (Robert et al., 2009;Sponaugle et al., 2010;Murphy et al., 2013). These considerations make it important to identify larval fish prey to the lowest taxonomic resolution possible. ...
... While relatively few studies have empirically demonstrated a relationship between feeding success and growth in larval fishes (Dower et al., 2009;Robert et al., 2014b;and Pepin et al., 2015), even fewer studies have investigated the relationship between growth and the contribution of preferred prey to feeding success (Robert et al., 2009;Sponaugle et al., 2010;Murphy et al., 2013). The paucity of knowledge of larval prey preferences from highly resolved taxonomical information, and how this absence of data may be affecting our capacity to predict recruitment, has been increasingly acknowledged in recent years (St. ...
... John et al., 2001;Robert et al., 2014a). A positive relationship between consumption of a larva's preferred prey and recent growth has been documented for fish species inhabiting various environments (Atlantic mackerel, Robert et al., 2009; Atlantic blue marlin [Makaira nigricans], Sponaugle et al., 2010;snapper [Chrysophrys auratus], Murphy et al., 2012). In both Atlantic mackerel and blue marlin, consumption of the preferred prey during early larval development, before the onset of piscivory, determined whether an individual would be characterized by fast or slow growth trajectory during the next stages of development (Robert et al., 2009;Sponaugle et al., 2010). ...
Year-class strength of fish stocks is often set during the larval stage, with fast growth being a favourable factor leading to strong recruitment. Following 30 years of poor recruitment, redfish (Sebastes sp.) in the Gulf of St. Lawrence (GSL) produced unprecedentedly strong year classes in 2011–2013. The relationship between larval diet and growth that potentially drove these successful recruitment events is unknown. Gut content and otolith microstructure of redfish larvae collected from the GSL in 1999 and 2000 were analysed in order to understand the relationship between recent feeding success and growth in larval redfish, and identify feeding behaviours that are associated with fast growth. Growth was positively correlated with larval body depth (BD), which was used as a morphological proxy for growth. Prey type and size were the best explanatory variables of larval BD after obligatory first-feeding. Larvae that consumed large naupliar stages of frequently consumed copepod taxa were deeper-bodied and grew more quickly than larvae that consumed redfish’s preferred prey, Calanus finmarchicus eggs.Warming GSL waters have shifted the phenology of commonly consumed prey taxa earlier in the season, whichmay increase the overlap between redfish and naupliar prey that drive fast growth, survival, and potentially recruitment success.
... A distinctive feature of this species was the occurrence of two microstructural zones in otoliths: a first zone where primary increments were more homogeneous and easy to identify, in contrast with a second zone where ring's alignment and other perturbations predominated. Microstructural zones in sagittal otoliths of YOY have been reported in several teleost fish (Wellington and Victor 1989;Beldade et al. 2007;Sponaugle et al. 2010;Garcia-Seoane et al. 2015), particularly in demersal and reef-associated fish (Victor 1986;Wilson and McCormick 1997;Sponaugle et al. 2010). These features have been linked roughly with the larval stage, the transition from larval to juvenile stages or habitat transitions, and later juvenile stages where juveniles occur in nursery areas or deeper habitats (Jenkins and May 1994;Raventos and Macpherson 2001;Plaza et al. 2001;Mansur et al. 2014). ...
... A distinctive feature of this species was the occurrence of two microstructural zones in otoliths: a first zone where primary increments were more homogeneous and easy to identify, in contrast with a second zone where ring's alignment and other perturbations predominated. Microstructural zones in sagittal otoliths of YOY have been reported in several teleost fish (Wellington and Victor 1989;Beldade et al. 2007;Sponaugle et al. 2010;Garcia-Seoane et al. 2015), particularly in demersal and reef-associated fish (Victor 1986;Wilson and McCormick 1997;Sponaugle et al. 2010). These features have been linked roughly with the larval stage, the transition from larval to juvenile stages or habitat transitions, and later juvenile stages where juveniles occur in nursery areas or deeper habitats (Jenkins and May 1994;Raventos and Macpherson 2001;Plaza et al. 2001;Mansur et al. 2014). ...
Age estimates were made on otoliths of 32 young-of-the-year (YOY) Barracuda, Sphyraena viridensis Cuvier (1829), captured in the south-western Mediterranean Sea. Increments were observed in Barracuda sagittae sectioned in a transverse plane and viewed with light microscopy. Increment counts were made for age estimation. Estimated ages ranged from 27 to 299 days of fish ranging in size from 15.9 to 38.9 cm total length (TL). The observed mean growth rate is 3.487 mm.day−1. Decreasing growth performance is evidenced according to age: 3.822 mm.day−1 between 159 and 302 mm (27–139 days), 1.596 mm.day−1 between 352 and 389 mm (172–299 days).
... Anderson, 1994;Robert et al., 2008;Voss et al., 2009;Demontigny et al., 2012;Murphy et al., 2012;Llopiz and Cowen, 2008). Successful feeding on preferred prey species generally results in high larval growth and survival (Robert et al., 2009;Sponaugle et al., 2010;Murphy et al., 2013). Given the importance of preferred prey taxa in driving variability in growth and survival potential, it is essential to investigate prey selectivity during the larval stage as a first step towards the robust assessment of recruitment-relevant bottom-up processes in a given population (Robert et al., 2014). ...
... There is increasing evidence that many fish species exhibit strong prey preference and selectivity during the larval stage (Robert et al., 2008). In selective species, larvae that feed successfully on preferred prey are characterized by faster growth, which increases their probability of survival (Robert et al., 2009;Sponaugle et al., 2010;Murphy et al., 2013). Various potential consumable prey taxa occur within a given zooplankton assemblage, but in order to optimize growth and survival, larvae should preferentially feed on taxa that are abundant, easily captured, and provide the highest net gain of energy and essential nutrients (Schoener, 1971;Werner and Hall, 1974). ...
Spatiotemporal overlap between fish larvae and their planktonic prey is an important source of recruitment variability. Over the past decade, one species of redfish, Sebastes mentella, from the Gulf of St. Lawrence (GSL) produced multiple strong cohorts following decades of low recruitment, which has generated strong interest in identifying potential drivers of larval survival. The present study provides the first detailed, multi-year assessment of larval redfish (Sebastes spp.) trophodynamics. Interannual variability in larval redfish diet composition and prey selectivity was assessed using high-resolution prey identification of larval gut contents and in situ prey fields. Eggs from the calanoid copepod Calanus finmarchicus represented the most frequently consumed prey in 3 of the 4 collection years, and contributed the largest proportion of carbon ingested by redfish larvae in all years. The high consumption of C. finmarchicus eggs by larvae, combined with evidence of positive selection for this taxon in some years, supports the hypothesis of a strong trophic link between larval redfish and a key calanoid copepod in the GSL ecosystem. Our results indicate that future efforts investigating GSL redfish recruitment processes should consider environment-driven variability in the reproductive phenology and abundance of C. finmarchicus.
... Nine species: yellowfin tuna (Thunnus albacares), blue marlin (Makaira nigricans), bigeye tuna (Thunnus obesus), white marlin (Kajikia albidus), swordfish (Xiphias gladius), sailfish (Istiophorus albicans), bluefin tuna (Thunnus thynnus), albacore tuna (Thunnus alalunga), and skipjack tuna (Katsuwonus pelamis) were each divided into two-stage multi-stanza groups (juvenile and adult) to account for ontogenetic changes in diet and fishing selectivity. Larval conspecifics were included in the juvenile life stage, as tuna and billfish larval stages are much shorter than one year and growth rates are rapid (Fromentin and Powers, 2005;Sponaugle et al., 2010). The age of each multi-stanza division was determined by the age at maturity referenced in stock assessments. ...
Ecosystem-based modeling is rapidly becoming an established technique to investigate the health and stability of ecosystems. In the Gulf of Mexico, ecosystem models are applied to neritic systems, but less focus has been placed on the oceanic domain. Since 2011, severe declines have been observed in many micronekton groups that occupy the mesopelagic zone (200-1000 m depth). Here we present an ecosystem model for the oceanic northern Gulf of Mexico for the year 2011, simulate that model according to micronekton trends through 2018, and quantify the top-down and bottom-up impacts that each functional group has on one another. These trends were examined to determine whether interactions between the two groups have changed directionally over time. In 2011, zooplankton (trophic level =2) occupied greater than one-third of the total metazoan biomass, and also 40% of the total energy throughput ascended to higher trophic levels in the system. Of the 1849 possible functional group interactions (most of which are indirect), approximately 27% showed significant changes between 2011 and 2018, which were related to shifts in biomass and diet throughout the simulation. Direct top-down interactions changed more frequently than other types of trophic relationships. The frequency of direct changes that occurred in the simulation was not observed evenly among all functional groups, as opposed to indirect interactions. These changes between functional group interactions can be used to further examine potential shifts in the trophic structure of marine ecosystems under various forcing scenarios.
... A low nutrient concentration from clean water discharge from the dam, working interactively with a low water temperature from hypolimnetic discharge, may induce low productivity and a shortage of food availability for fish (Garnier et al. 2002;Sponaugle et al. 2010). No research had been conducted investigating impacts of dam release on primary productivity and plankton downstream the TGD. ...
Drifting larval fish assemblages were investigated at three sections in the middle reach of the Yangtze River downstream of the Three Gorges Dam to reveal recovery gradients and to identify the major environmental factors influencing their temporal and spatial patterns. Larval fish in the river section closest to the dam showed lower abundance, later occurrence, narrower temporal niche breadth, and more pairs of dominant species with high niche overlap than those further from the dam. Water temperature, transparency, and discharge significantly influenced the larval fish assemblages. They showed longitudinal gradients of increased water temperature, decreased transparency, and increased discharge in a downstream direction. Our results suggested that hypolimnetic and clean water discharge from the dam heavily impacted larval fish assemblage in the river section near the dam. With an increased distance to the dam and the input of some major tributaries, this impact was gradually buffered, thus forming the longitudinal recovery gradients. We suggest that priority for conservation should shift from river sections closer to the dam to those further from the dam.
... Nine species: yellowfin tuna (Thunnus albacares), blue marlin (Makaira nigricans), bigeye tuna (Thunnus obesus), white marlin (Kajikia albidus), swordfish (Xiphias gladius), sailfish (Istiophorus albicans), bluefin tuna (Thunnus thynnus), albacore tuna (Thunnus alalunga), and skipjack tuna (Katsuwonus pelamis) were each divided into two-stage multi-stanza groups (juvenile and adult) to account for ontogenetic changes in diet and fishing selectivity. Larval conspecifics were included in the juvenile life stage, as tuna and billfish larval stages are much shorter than one year and growth rates are rapid (Fromentin and Powers, 2005;Sponaugle et al., 2010). The age of each multi-stanza division was determined by the age at maturity referenced in stock assessments. ...
Although they likely dominate the world’s total fish biomass, the roles of mesopelagic fishes in oceanic ecosystems are poorly represented in ecosystem models. Mesopelagic fishes are occasionally practiced as commercial species, but restrictions due to fishing cost and gear selectivity have stunted global production. Many mesopelagic fishes undertake a diel vertical migration where they ascend to the near-surface waters during the night to feed and descend into the depths during the day. Other mesopelagic fishes remain at depth throughout the diel cycle. Apex predators (e.g., tunas, marine mammals, sharks) make routine dives to prey on micronekton in the mesopelagic zone (200 – 1000 m depth). Here we introduce an ecosystem model of the oceanic Gulf of Mexico and emphasize the trophic impact that mesopelagic fishes have on other functional groups within the system. This model has been coupled with a biogeochemical model (ROMS-NPZD), compared to 39 time series, and validated with a Monte Carlo procedure. Mesopelagic fishes occupy a secondary consumer role in the ecosystem (trophic level 3.3 – 4.0), where feeding guilds range from zooplanktivores to micronektonivores. In total, mesopelagic fishes occupy greater than 80% of the total energy pathways that progress from primary producers to apex predators. Lanternfishes are an important prey species for juvenile tunas and dragonfishes. Based on predator and prey groups, hatchetfishes and lanternfishes both occupy a similar niche to pelagic cephalopods (another important prey group to apex predators). Mesopelagic fishes are prey to upper-trophic level organisms and consume zooplankton, potentially providing wasp-waist control in oceanic ecosystems.
... Within a population, larval growth can vary spatially (e.g. Suthers et al., 1989;Sponaugle et al., 2010) and interannually (e.g. Olafsdottir and Anderson, 2010;Murphy et al., 2013;Rodríguez-Valentino et al., 2015;Takahashi et al., 2016). ...
We examined larval capelin density and growth dynamics in two relatively unstudied northern coastal embayments of Newfoundland (White Bay and Notre Dame Bay), comparing these larval characteristics to those measured in the annually-surveyed Trinity Bay in order to assess the spatiotemporal variability in larval dynamics and its potential implications on subsequent recruitment. We conducted ichthyoplankton surveys in August of 2015 and 2016, assessing larval density and using otolith microstructure analyses to estimate larval age and growth rates. Size of larvae captured over the two years ranged between 4 and 17 mm, corresponding to an estimated age of 1–33 d. Our results indicated substantial spatial and interannual variability in both density and growth. Larval density was similar between bays in 2015 but drastically different in 2016, where the northern bays were characterized by an order of magnitude less larvae than Trinity Bay. Larval growth was significantly higher in the northern bays in 2015, but lower in 2016, relative to Trinity Bay. This spatiotemporal variability in growth, abundance, and survival potential of larval capelin indicates that the proportion of recruits originating from these key spawning areas fluctuates interannually, with potential implications for the assessment and management of the stock.
Restricted to low-productivity environments near their thermal maxima, larval tunas may be threatened by warming global temperatures, yet our understanding of how they are constrained is limited. We examined blackfin tuna (Thunnus atlanticus, presumptive) diet and growth in the context of their prey and predators in the Straits of Florida in 2 years with contrasting summer conditions: low temperature (26.7–28.3°C)–high prey and high temperature (28.4–29.0°C)–low prey. In the cooler, high-prey conditions, larvae had 30% faster growth (0.45 mm d−1), fuller guts from predominantly feeding on calanoid copepods, and were 10× more abundant, dominating the larval fish assemblage. In contrast, in warm, low-prey conditions fewer, younger, and slower-growing (0.35 mm d−1) T. atlanticus fed predominantly on nauplii and had less full guts. Modelling individual growth across years revealed that growth peaked at an optimum of 28.5°C (5°C below known thermal maxima in the field) and high densities of predators selectively consumed slower-growing larvae. Low-prey availability may reduce the thermal optima of larvae, as growth and survival are diminished when low prey and high temperature coincide. Our results illustrate the importance of considering food web dynamics with temperature when predicting the response of organisms to ecosystem variability, particularly ongoing climate change.
Kawakawa tuna (Euthynnus affinis) is a novel target species for aquaculture in Japan and currently there is very limited knowledge on its early ontogenetic development in captivity. The ontogeny of E. affinis larvae from hatching to 20 days after hatching (DAH) was illustrated in the present study through an integrated approach of histology, biochemistry and molecular approaches. Larvae were fed enriched L-type rotifer Brachionus plicatilis species complex from 2-10 DAH, followed by enriched Artemia nauplii. Target digestive enzymes; trypsin, pepsin, lipase and amylase were assessed in order to evaluate nutritional condition. The larvae began exogenous feeding from 2 DAH, coinciding with the opening of the oral capacity and anus. The yolk sac was rapidly consumed and completely resorbed at 3 DAH, while the oil globule disappeared by 5 DAH. Enzymatic activity and expression of the target enzymes were detectable at hatching, except pepsin. With the development of the gastric glands, pepsin activity was then detected at 10 DAH, which was followed by rapid increase in pepsin activity at 15 DAH. The larvae showed rapid growth, however, high mortality also occurred at 6-10 DAH which has been highlighted as a critical stage in the larviculture of E. affinis. From 15-20 DAH, the histological structures of the digestive tract were completely segmented with the digestive tract resembling that of its juvenile stages. The activity of trypsin, pepsin, and lipase tended to increase sharply, while amylase dramatically dropped, possibly indicating a change in dietary preference. Findings from the present study provides a preliminary insight into the early development of the digestive system and associated digestive enzymes of E. affinis larvae and establishes an initial step towards the rearing and feeding protocol optimization of this novel target species.
Survival of fish in the first year is critical in determining the year‐class strength of fish populations. In river–floodplain ecosystems, affiliated lakes (i.e., floodplain lakes that are hydrologically connected to the river) play a crucial role as nursery habitats for young‐of‐the‐year (age‐0) fish. However, most lakes in the river–floodplain ecosystem of the Yangtze River, China, have been disconnected. Data on age‐0 fish distribution are important for developing appropriate strategies for management and conservation of fish populations in the river. We investigated the distribution patterns of age‐0 fish in the middle reaches of the Yangtze River. Overall, 33 species were collected from four main‐stem reaches (29 species) and two lakes (32 species) connected with the main stem from July to September in 2010 and 2011. The CPUE and species richness of age‐0 fish were higher in the lakes; age‐0 species richness showed an increasing trend in a downstream direction along the main stem. Analysis of similarities revealed that significant differences in assemblage structure of age‐0 fish occurred mainly among sampling sites. The higher CPUE and species richness of age‐0 fish in the lakes indicate the importance of connected lakes as age‐0 fish nursery habitats. We also found abundant age‐0 fish in the main stem and a relationship between spatial patterns of age‐0 fish with site proximity to the Three Gorges Dam, similar to previously observed spatial patterns in larval fishes. These results indicate the importance of the main stem as age‐0 fish nursery habitat and the effects of the dam on age‐0 fish distribution. This study presents the first data on age‐0 fish in the Yangtze River, thus providing a baseline for future age‐0 fish research. It also provides essential information for management and conservation measures, such as restoring the lateral connectivity between the main stem and disconnected lakes and guidelines for ecological operation of the dam.
Lengths and ages of swordfish (Xiphias gladius) estimated from increments on otoliths of larvae collected in the Caribbean Sea, Florida Straits, and off the southeastern United States, indicated two growth phases. Larvae complete yolk and oil globule absorption 5 to 6 days after hatching (DAH). Larvae <13 mm preserved standard length (PSL) grow slowly (∼0.3 mm/d); larvae from 13 to 115 mm PSL grow rapidly (∼6 mm/d). The acceleration in growth rate at 13 days follows an abrupt (within 3 days) change in diet, and in jaw and alimentary canal structure. The diet of swordfish larvae is limited. Larvae <8 mm PSL from the Caribbean, Gulf of Mexico, and off the southeastern United States eat exclusively copepods, primarily of one genus, Corycaeus. Larvae 9 to 11 mm eat copepods and chaetognaths; larvae >11 mm eat exclusively neustonic fish larvae. This diet indicates that young larvae <11 mm occupy the near-surface pelagia, whereas, older and longer larvae are neustonic. Spawning dates for larvae collected in various regions of the western North Atlantic, along with the abundance and spatial distribution of the youngest larvae, indicate that spawning peaks in three seasons and in five regions. Swordfish spawn in the Caribbean Sea, or possibly to the east, in winter, and in the western Gulf of Mexico in spring. Elsewhere swordfish spawn year-round, but spawning peaks in the spring in the north-central Gulf of Mexico, in the summer off southern Florida, and in the spring and early summer off the southeastern United States. The western Gulf Stream frontal zone is the focus of spawning off the southeastern coast of the United States, whereas spawning in the Gulf of Mexico seems to be focused in the vicinity of the Gulf Loop Current. Larvae may use the Gulf of Mexico and the outer continental shelf off the east coast of the United States as nursery areas. Some larvae may be transported northward, but trans-Atlantic transport of larvae is unlikely.
Tuna larvae (at flexion, postflexion, and transformation stages) were collected by dip net and light traps at night in the northwestern Panama Bight during the season of reduced upwelling (June-September) of 1990, 1991, 1992, and 1997. The larvae were identified as yellowfin tuna (Thunnus albacares) by mtDNA analysis. Ichthyoplankton data from bongo and Tucker trawl tows were used to examine the potential prey abundance in relation to the mean size-at-age and growth rates of the yellowfin tuna larvae and their otoliths. The most rapid growth rates occurred during June 1990 when plankton volumes were at their highest levels. The lowest plankton volumes coincided with the lowest growth rates and mean sizes-at-age during the August-September 1991 period. High densities of larval fish were prevalent in the ichthyoplankton tows during the 1991 period; therefore intra- and interspecific competition for limited food resources may have been the cause of slower growth (density-dependent growth) in yellowfin tuna larvae The highest mean sea-surface temperature and the lowest mean wind stress occurred during an El Niño-Southern Oscillation (ENSO) event during the 1997 period. There appeared to be no clear association between these environmental factors and larval growth rates, but the higher temperatures may have caused an increase in the short-term growth of otoliths in relation to larval fish size.
The objective of this dissertation was to examine sailfish ( Istiophorus platypterus) and blue marlin (Makaira nigricans ) spawning in the Straits of Florida, with a specific focus on (1) the physical and biological characteristics of the spawning environment, and (2) the role of the region within the broader spawning patterns of these two species. In order to accomplish these objectives, two years of monthly ichthyoplankton collections and physical measurements across the Straits of Florida were combined with a finer-scale Lagrangian study. Additionally, a molecular species-identification methodology was developed that was both high-throughput and suitable for use with a broad taxonomic range of species.An initial analysis considered the diversity, assemblages and associated habitat of the larvae of large and medium size pelagic species. In total 36 species and 14,295 individuals were collected during this study, with the highest diversity occurring during the summer, and in the western frontal region of the Florida Current. Sailfish were included in an assemblage with Auxis rochei, A. thazard and Euthynnus alleterattus, all species found in highest abundance during the summer along the western edge of the Straits of Florida. Blue marlin grouped most closely with Thunnus atlanticus, Ruvettus pretiosus and Lampris guttatus, all summer spawners, whose larvae tended to occur further offshore. The primary environmental factors associated with these assemblages were SST (highest summer-early fall), day-length (highest early summer), thermocline depth (shallowest on the Florida side) and fluorescence (highest on the Florida side).A Lagrangian sampling effort was then used to more specifically evaluate the role of frontal zones in sailfish spawning. The results of this sampling indicated that the highest levels of sailfish spawning occurred in a frontal zone associated with the formation of a submesoscale frontal eddy. This spawning resulted in the first-feeding larvae occupying an area rich in prey items. Given the small spatial-scale of the front, and the distribution of the eggs of adult prey items, the results of this work would suggest that sailfish are actively targeting features for spawning that are favorable to the growth and survival of their larvae.Finally the relative importance of the Straits of Florida as a spawning ground was evaluated by calculating the annual egg production of both sailfish and blue marlin within this region. In total it was estimated that 2.1% of western Atlantic sailfish spawning and 1.6% of Atlantic wide blue marlin spawning occurs in the SF. Pop-up satellite tags deployed on sailfish at the start of the spawning season revealed their short residency times in the SF, suggesting that a large (≈10%) transient portion of the sailfish population is responsible for the SF egg production. These results indicate that the SF is a migratory bottleneck for sailfish.In conclusion the results of this study indicate that a hierarchy of physical and biological processes influence the distribution of billfish spawning in space and time. The results provide insights into the movement patterns and life history strategies of these species, and ultimately may aid in the development of the spatially explicit ecosystem based management approaches that are currently being advocated.
Zooplankton availability is strongly linked with the growth, survival, and ultimately recruitment of fish during their early life history. We examined how different types of zooplankton affected the growth, survival, and prey selection of larval yellow perch Perca flavescens in a series of laboratory experiments. The growth and survival of newly hatched (5–7-mm) to 12-mm yellow perch larvae was greatest when feeding on adult copepods and copepod nauplii. The growth and survival of yellow perch larvae longer than 12 mm was greatest when feeding on adult copepods and small cladocerans. Prey selection patterns closely followed trends in growth and survival; smaller larvae positively selected adult and naupliar copepods, whereas larger larvae selected adult copepods and small cladocerans. Foraging behavior experiments conducted with larvae longer than 12 mm revealed that these fish derived similar energetic gains when feeding solely on adult copepods and small cladocerans. The pathway to this energetic benefit differed substantially between prey types. Adult copepods yielded substantial energy to larval yellow perch because of the minimal handling time involved, despite the lower capture efficiency than with cladocerans. Conversely, yellow perch larvae realized high energetic gain from small cladocerans because of high capture efficiency, despite the higher handling times than with adult copepods. These results illustrate the importance of experimentally quantifying the feeding behavior of fish larvae to gain insight into how larval fish behavior and food type interact to shape larval fish growth patterns.
To estimate the survival process of Pacific bluefin tuna Thunnus orientalis during the larval period, estimated growth histories were compared between larvae collected in late spring and juveniles collected in the boreal summer of 2004, which were considered to be survivors of the larval cohorts. Larval tuna (3.3 to 9.6 mm standard length, SL) were collected from mid-May to early June around the Ryukyu Islands, northwestern Pacific Ocean, and juvenile tuna were collected offshore of Kochi and Nagasaki prefectures in July-August. Preflexion, flexion and postflexion larvae were collected, and their ages ranged from 4 to 18 d. Back-calculated SLs by the biological intercept method showed that larval tuna in the postflexion phase were larger-at-age than preflexion and flexion larvae, suggesting that only larger and faster growing larvae were able to survive to the postflexion phase. The logarithms of otolith radii (In OR: proportional to SL) of larvae with slower growth and development were smaller than the minimum In OR of surviving juvenile tuna, which indicated the smallest possible size required for larvae to successfully recruit to the fishery. These results indicate that the survival of larvae of Pacific bluefin tuna depends largely on size and growth rates during early life history.
The identification of larval istiophorid billfishes from the western North Atlantic Ocean has long been problematic. In the present study, a molecular technique was used to positively identify 27 larval white marlin (Tetrapturus albidus), 96 larval blue marlin (Makaira nigricans), and 591 larval sailfish (Istiophorus platypterus) from the Straits of Florida and the Bahamas. Nine morphometric measurements were taken for a subset of larvae (species known), and lower jaw pigment patterns were recorded on a grid. Canonical variates analysis (CVA) was used to reveal the extent to which the combination of morphometric, pigment pattern, and month of capture information was diagnostic to species level. Linear regression revealed species specific relationships between the ratio of snout length to eye orbit diameter and standard length (SL). Confidence limits about these relationships served as defining characters for sailfish >10 mm SL and for blue and white marlin >17 mm SL. Pigment pattern analysis indicated that 40% of the preflexion blue marlin examined possessed a characteristic lower jaw pigment pattern and that 62% of sailfish larvae were identifiable by lower jaw pigments alone. An identification key was constructed based on pigment patterns, month of capture, and relationships between SL and the ratio of snout length to eye orbit diameter. The key yielded identifications for 69.4% of 304 (blind sample) larvae used to test it; only one of these identifications was incorrect. Of the 93 larvae that could not be identified by the key, 71 (76.3%) were correctly identified with CVA. Although identification of certain larval specimens may always require molecular techniques, it is encouraging that the majority (92.4%) of istiophorid larvae examined were ultimately identifiable from external characteristics alone.
Onshore transport of new nitrogen from the nutrient-bearing strata beneath the Gulf Stream indicates that frontal eddies serve as a "nutrient pump' for the shelf. New nitrogen flux to the shelf due to Gulf Stream input could support new production of 7.4 × 1012 g C yr -1 or c8 million tons carbon per year if all nitrate were utilized. Approximately 70% of this potential new production is realized, yielding an annual new production for the outer shelf of 4.3 × 1012g C. -from Authors
Monthly plankton sampling across the Straits of Florida (SOF) allowed for a thorough investigation of the feeding ecologies of four taxa of larval tunas (family Scombridae, tribe Thunnini) and the horizontal and vertical distributions of tuna larvae and their dominant prey. Before piscivory, Thunnus spp. larvae had a mixed diet of crustaceans and appendicularians, whereas skipjack tuna (Katsuwonus pelamis), little tunny (Euthynnus alletteratus), and Auxis spp. displayed highly selective and nearly exclusive feeding on appendicularians. The availability of both appendicularians and larval fish prey declined from west to east across the SOF, and appendicularians were notably patchy. In the western SOF where prey was more abundant, all taxa of tuna larvae co-occurred, indicating the sharing of resources by the larvae, in addition to the adults of these taxa using similar spawning habitat upstream in the Florida Current. In the central and eastern SOF, where prey was less abundant, only Thunnus spp. and skipjack tuna co-occurred, and these two taxa exhibited significantly different vertical distributions. Prey removal rates (estimated from gut evacuation rates and daily rations) occurring in the western SOF where tuna taxa co-occurred are likely to be sustainable by appendicularian levels within this region but would potentially not be by levels in the east. The spatial and trophic characteristics of these four abundant larval taxa highlight the potential influence of feeding-related processes on larval and adult behavior, while also illustrating a critical trophic link to the microbial food web provided by appendicularians in this oligotrophic environment.
Scombrid fishes are considered to have adopted a survival strategy characterized by fast growth and the ability to consume large prey at an early age. Variability in feeding condition can significantly affect larval growth as early as the post first-feeding stage and may control growth-related survival during the early larval stage. Through otolith microstructure analysis, growth-selective survival was demonstrated for post first-feeding larvae of Japanese Spanish mackerel Scomberomorus niphonius, which exhibit exclusive piscivory, a high growth rate, and low tolerance to starvation. Larvae and juveniles were repeatedly collected in the Seto Inland Sea in June and July from 1997 to 1999. Growth trajectory back-calculated using otolith microanalysis was compared between the survivors (SV) captured in July and the presumed original population (OP) captured in June. Selection for fast-growing larvae was evident during the 5 d after first feeding (Days 5 to 10). More than 90% of the SV had a mean growth rate during Days 5 to 10 (G(5-10)) > 0.8 mm d(-1), and those with G5-10 < 0.8 mm. d(-1) rarely survived the period. Intensity of selection differed among 3 years. A less intense selection was observed in 1999 when the larval cohort experienced higher prey concentrations and G(5-10). Laboratory experiments demonstrated that 1 to 2 d starvation retarded the larval growth during the following larval period, and supported the conclusion that fluctuation in ichthyoplankton prey abundance alters larval survival probability by affecting the larval growth rate within a short period subsequent to first feeding.