Age-linked changes in salinity tolerance of larval spotted seatrout (Cynoscion nebulosus, Cuvier)

Acute salinity tolerance limits for the estuarine spawning spotted seatrout, Cynoscion nebulosus (Cuvier). were evaluated by examining 18 h survival of larvae in an extensive range of salinity treatments (0 to 56 ppt). Larvae from eggs spawned in two different salinities (24 and 32 ppt) as well as larvae acclimated in hypersaline and brackish waters were compared. Both upper and lower salinity tolerance limits showed an age-linked pattern, decreasing to a minimum tolerance range (6.4 to 42.5 ppt) at age 3 days after hatching (at 28 o C) and increasing to the widest range tolerated (1.9 to 49.8 ppt) on the last day tested (age 9 days). Acclimation to hyposaline conditions was demonstrated by larvae spawned at 32 ppt although significant hypersaline acclimation could not be demonstrated. Altered upper limits to the range tolerated by larvae from different spawning salinities indicated parental and/or early acclimation effects are important. Consistently greater vulnerability to both hyper- and hyposaline conditions at age 3 days after hatching was observed in all tests conducted. Exposures related to the onset of feeding at this time are likely explanations for this reduced tolerance.     

Feeding performance of juvenile hatchery‐reared spotted seatrout Cynoscion nebulosus

The feeding performance of individual hatchery‐reared (HR) and wild juvenile spotted seatrout Cynoscion nebulosus was compared across a series of six 1·5 h feeding exposures over a 3 day period in a controlled experiment. The predation cycle served as a context for discerning feeding performance elements. The experimental design facilitated assessments of the effects of experience, motivation due to hunger or satiation and prey density and encounter frequency. Although feeding success improved significantly across successive trials for both groups of C. nebulosus, wild C. nebulosus successfully captured and consumed significantly more Palaemonetes spp. prey and completed most performance metrics more efficiently than HR C. nebulosus. Total exposure time decreased with experience for both groups of C. nebulosus; however, HR C. nebulosus took longer to complete feeding exposures. Underpinning this difference was the time spent by HR C. nebulosus in non‐search mode and for completing various foraging behaviours. Nevertheless, juvenile HR C. nebulosus exhibited sufficient foraging plasticity to switch from a pelleted diet to live novel prey.     

Clinal variation at aspartate aminotransferase-2 in spotted seatrout,Cynoscion nebulosus (Cuvier), inhabiting the north-western Gulf of Mexico

Clinal variation was observed at the aspartate aminotransferase-2 (AAT-2) locus in the marine fish Cynoscion nebulosus (Cuvier) inhabiting the bays and estuaries of the Texas and northern Mexico Gulf coasts. Frequency of the AAT-2(80) allele increased from 0.9% at Sabine Lake, Texas to 17.1% at Rio Soto La Marina, Mexico. A statistically significant correlation existed in the frequency of this allele with degrees north latitude and west longitude. This information, if properly incorporated into a comprehensive enhancement programme, could facilitate supplemental stocking success.     

Spatial and temporal variability of juvenile spotted seatrout Cynoscion nebulosus growth in Chesapeake Bay

Juvenile spotted seatrout Cynoscion nebulosus growth in Chesapeake Bay was compared to growth from southerly estuaries to examine the potential for countergradient growth variation, and finer scale growth variation within Chesapeake Bay was investigated further. Because spawning and growing seasons in Chesapeake Bay are much shorter than in southern populations, and the bay population is genetically distinct, it was expected that C. nebulosus in Chesapeake Bay may exhibit faster juvenile growth than their southern counterparts. Within the bay, spatial and temporal growth patterns were examined using a repeated measures linear mixed-effects model on individual retrospective growth histories. Juvenile C. nebulosus were collected from seagrass beds throughout Chesapeake Bay in 1997–1999 and 2002; sagittae were removed for daily growth analysis. The calculated growth rate of 1·44 mm standard length day⁻¹ for Chesapeake Bay fish is two to three times that reported for Florida C. nebulosus . Within the bay from 1997 to 1999 (average rainfall years), growth patterns were similar with the fastest growing fish collected from seagrass beds in the central bay, followed by the eastern shore fish with intermediate growth and western shore fish that exhibited slowest growth. These results were reversed (western shore fish growth > eastern shore > central bay) in 2002 (a drought year) even though temperatures across all years were similar throughout the bay, indicating that growth may be influenced by freshwater inflows.     

The measurement of plasma vitellogenin levels in a marine teleost, the spotted seatrout (Cynoscion nebulosus) by homologous radioimmunoassay

1. Vitellogenin was purified from plasma of estrogen-treated spotted seatrout (Cynoscion nebulosus) and was shown to be a protein of approximately 200,000 mol. wt. 2. Antisera were raised against the isolated protein and were used to develop a radioimmunoassay for spotted seatrout vitellogenin. 3. The substance measured was shown to be female-specific, hepatically-derived, estrogen-inducible and immunologically related to oocyte yolk proteins. 4. Vitellogenin levels in the plasma of female spotted seatrout were shown to rise dramatically during the period of ovarian recrudescence.     

Spawning site selection by spotted seatrout,Cynoscion nebulosus,and black drum,Pogonias cromis,in Louisiana

Synopsis Spawning site selection by spotted seatrout and black drum was studied by locating drumming aggregations through the use of a hydrophone. From March 1987 to October 1990, 315 sound observations were made to identify and characterize spawning seasons and environmental requirements of both species in the Barataria, Caminada, and eastern Timbalier Bay systems of Louisiana. The sounds produced by the spawning aggregations were identified and verified against known recordings. Spawning was verified on several occasions by capturing and rearing zygotes (eggs) into identifiable larvae. Spotted seatrout formed drumming aggregations from late May to early October at salinities ranging from 7.0 to 25.8 ppt and temperatures from 24.5 to 33.5° C. Black drum formed drumming aggregations between January and April in salinities from 10.0 to 27.0 ppt and temperatures from 15.0 to 24.0°C. Large drumming aggregations of spotted seatrout were located from 1800 to 2400h and from 1800 to 2200h for black drum. Spotted seatrout aggregation size was highly correlated with water temperature and for black drum with dissolved oxygen concentrations. Spawning sites for both species were frequently located in deep moving water between barrier islands as well as in channels in open water where water depth ranged from 3 to 50 m. Spawning site selection depended on a particular range of environmental conditions and spawning locations varied seasonally and yearly depending upon hydrological variation.Correspondence to D.M. Baltz     

Distribution of serum transferrin in spot, Leiostomus xanthurus

Serum transferrin distribution was studied in 128 spot, Leiostomus xanthurus, obtained from the lower Rappahannock River of Virginia. Three phenotypes designated as TfA, TfB and TfAB were observed. These are described as representing two codominant alleles at a single gene locus. Phenotypic distribution did not differ significantly from that predicted by Hardy-Weinberg analysis.     

Genetic population structure of spotted seatrout Cynoscion nebulosus along the south‐eastern U.S.A.

Analyses of the genetic population structure of spotted seatrout Cynoscion nebulosus along the south‐eastern U.S. coast using 13 microsatellites suggest significant population differentiation between fish in North Carolina (NC) compared with South Carolina (SC) and Georgia (GA), with New River, NC, serving as an area of integration between northern and southern C. nebulosus. Although there is a significant break in gene flow between these areas, the overall pattern throughout the sampling range represents a gradient in genetic diversification with the degree of geographic separation. Latitudinal distance and estuarine density appear to be main drivers in the genetic differentiation of C. nebulosus along the south‐eastern U.S. coast. The isolation‐by‐distance gene‐flow pattern creates fine‐scale differences in the genetic composition of proximal estuaries and dictates that stocking must be confined to within 100 km of the location of broodstock collection in order to maintain the natural gradient of genetic variation along the south‐eastern U.S. coast.     

Susceptibility and tolerance of spotted seatrout, Cynoscion nebulosus , and red snapper, Lutjanus campechanus , to experimental infections with Amyloodinium ocellatum

Amyloodinium ocellatum is a parasitic dinoflagellate that infects warm-water marine and estuarine fishes and causes mortalities in aquaculture. Its life cycle consists of 3 stages: a feeding trophont that parasitizes the gills and skin where it interferes with gas exchange, osmoregulation, and tissue integrity; a detached reproductive tomont; and a free-swimming infective dinospore. We compared the susceptibility and tolerance of juvenile spotted seatrout, Cynoscion nebulosus, and red snapper, Lutjanus campechanus, to this parasite by individually exposing fish in 3-L aquaria (at 25 C and 33 practical salinity units) to several dinospore doses over different time periods and quantified the size and number of resulting trophonts. We estimated the trophont detachment rate and trophont size at detachment, the 24-hr dinospore infection rate, the dinospore 48-hr median lethal dose (LD(50)), and the trophont lethal load at the 48-hr LD(50). There were no significant differences in dinospore infection rates or dinospore lethal doses between spotted seatrout and red snapper; however, trophonts remained attached longer and attained a larger size in red snapper than in spotted seatrout. The trophont lethal load was significantly higher in spotted seatrout than in red snapper. A proposed model simulating the trophont dynamics reflected our experimental findings and showed that A. ocellatum reproductive success is linked both to the number of dinospores and the size of the trophont, factors that, in turn, are linked to the time the trophont spends on the host and the number of trophonts the host can tolerate.     

Effects of release procedures on the primary stress response and post‐release survival and growth of hatchery‐reared spotted seatrout Cynoscion nebulosus

To help explain the apparent poor post‐release success of hatchery‐reared (HR) spotted seatrout Cynoscion nebulosus, this study examined the effects of handling, transport and release procedures on the stress response of two age classes [48 and 80 day post‐hatch (dph)] of HR C. nebulosus, as measured by cortisol concentrations and the post‐release survival and growth of 48 and 80 dph HR C. nebulosus. As a proxy for stress, tissue cortisol was measured at various times during the handling, tagging (80 dph), transport, acclimation and release process. To consider the implications of the pre‐release stressors, growth and survival were monitored in separate field experiments for each age class of acclimated post‐transport C. nebulosus using control C. nebulosus that only experienced anaesthesia, transport, acclimation and a net release v. experimental C. nebulosus that underwent the entire routine procedure, including anaesthesia, tagging, transport, acclimation and gravity release through a pipe. For 48 dph C. nebulosus, mean cortisol varied significantly throughout handling and transport, increasing more than six‐fold from controls before decreasing in mean concentration just prior to release. For 80 dph C. nebulosus, cortisol varied throughout handling, tagging and transport, first increasing more than three‐fold compared with control C. nebulosus, before decreasing and rising slightly just prior to release. For 48 dph C. nebulosus within field enclosures, survival was high and similar for control and experimental groups; experimental C. nebulosus, however, were shorter, lighter and lower in condition than control C. nebulosus. For 80 dph C. nebulosus within field enclosures, fewer experimental C. nebulosus survived and those that did survive were of lower condition than C. nebulosus from the control group. Small untagged C. nebulosus may survive the release procedure better than larger C. nebulosus carrying a coded‐wire tag. These findings document some ways in which pre‐release practices may translate into detrimental effects on post‐release success of HR C. nebulosus.     

Base and stressed ventilation rates for Leiostomus xanthurus Lacépède and Morone americana (Gmelin) exposed to strobe lights

A biomonitoring system interfaced with a microcomputer was used to monitor ventilation rates for white perch (Morone americana) and spot (Leiostomus xanthurus) under baseline and stressed conditions caused by strobe lights. Tests were conducted on light‐ and dark‐acclimated specimens. These two estuarine species have been found to exhibit avoidance behavior to strobe lights. Potential accommodation to the strobe light stimulus was explored over a 24 h period. The biomonitoring system successfully recorded ventilation rates under baseline and stressed conditions. Baseline mean ventilation rates for 0.5 h intervals ranged from 1 count per minute (cpm) to 97 cpm for light‐acclimated white perch with an overall mean for 24 h (x ) of 41 cpm. Mean stressed rates ranged from 1 to 100 cpm with an overall mean of 44 cpm. Baseline rates for dark‐acclimated white perch ranged from 1 to 79 cpm (x = 35 cpm), with stressed rates from 2 to 83 cpm (x = 30 cpm). Light‐acclimated spot had baseline ventilation rates ranging from 3 to 146 cpm (x = 42 cpm), while stressed rates ranged from 2 to 134 cpm (x = 36 cpm). Mean baseline rates for dark‐acclimated spot ranged from 1 to 94 cpm (x = 40 cpm), and stressed rates ranged from 1 to 72 cpm (x = 25 cpm). The difference in ventilation rates between base and stressed conditions (as absolute values) for light‐acclimated white perch over the 24 h experiments ranged from 0 to 43 cpm (x = 11.01 cpm). Dark‐acclimated white perch had differences ranging from 0 to 78 cpm (x = 11.13 cpm). Light‐acclimated spot had differences ranging from 0 to 101 cpm (x = 14.68 cpm). Dark‐acclimated spot had differences ranging from 0 to 70 cpm (x = 20.56 cpm). Ventilation rates varied between species and among individuals within a species. Ventilation rates were generally lower for dark‐acclimated specimens. For both species under all conditions, the base and stressed rates were significantly (P < 0.05) different during the 24 h period. However, dark‐acclimated specimens exhibited a more distinct difference than light‐acclimated specimens. The lack of accommodation to strobe light and a stronger reaction under dark conditions indicate that strobe lights continue to offer potential as behavioral guidance systems for these species.     

Community ecology of the parasites of adult spot, Leiostomus xanthurus, and Atlantic croaker, Micropogonias undulatus (Sciaenidae) in the Cape Hatteras region

Twenty-three species of metazoan parasites were recorded from adult spot and 26 from adult croaker. Of the 33 parasitic species found, 17 occurred in both spot and croaker. No significant differences in intensity of parasites occurred between sexes of either spot or croaker. All of the parasites had over-dispersed, or clumped, distributions among hosts. Adult spot and croaker collected offshore had much greater species-richness, diversity, and total number of individual parasites than juvenile fishes collected in adjoining estuaries during another study. The juvenile spot and croaker had less time to acquire parasites and inhabited less diverse and more confined habitats in inshore estuaries, which resulted in less diverse parasite communities than offshore fishes. The number of species and diversity of parasites in adult fish was greater in croaker than spot. However, when only gastrointestinal helminths were considered, spot had greater species-richness as well as greater numbers of individual helminths, suggesting that they had a more diverse diet and that they fed on more infected intermediate hosts than croaker. In both adult spot and croaker the mean number of parasitic species was greater than those of freshwater fishes and fewer than those for birds and mammals. The total number of individual parasites was similar to that of freshwater fishes. The opportunistic diet and the migratory habits of both spot and croaker contribute to their diverse parasite faunas. Comparison of adult spot and croaker parasite faunas collected offshore indicated that their respective parasite component communities were distinct and that similar parasite infracommunity variability existed in both hosts and that their communities were not ‘random’ samples, but restricted subsets of the compound community. Although the parasite dominance hierarchy in adults of both species varied slightly between areas and seasons sampled, there appeared to be a predictable dominant species that was accompanied by subordinate, less predictable species. However, the variability in both relative intensities and presence-absence of parasites within communities resulting from their diverse diets make them less predictable than those of other vertebrates with less diverse diets such as the lesser scaup duck and more like those of other marine fishes.     

A New Species of Kudoa (Myxosporidea: Chloromyxidae) from the Spot,Leiostomus xanthurus Lacépède,in Clear Lake,Texas*

SYNOPSIS. Kudoa branchiata sp. n. (Myxosporidea: Chloromyxidae) is described from the gills of the marine sciaenid fish, Leiostomus xanthurus Lacépède, from Clear Lake, Texas. Twelve of the 429 hosts examined from 10 September 1970 to 28 April 1971, were infected. Eight of the 12 infected fish were collected in February and March 1971—a period during which only 13% of the total host sample was taken. The mean total length of infected hosts was 149 mm, with a range of 112–185 mm. The mean number of myxosporidean cysts per infected host was 3.5, with a range of 1–11.     

The effect of spawning salinity on eggs of spotted seatrout (Cynoscion nebulosus, Cuvier) from two bays with historically different salinity regimes

Spotted seatrout are capable of spawning in a wide range of salinities. Along the Texas Gulf Coast, bay salinities increase from an average of 14 ppt in Galveston Bay to an average of 40 ppt in Lower Laguna Madre due to the negative gradient of freshwater inflow from north to south. Tagging studies have shown that the majority of spotted seatrout do not migrate between adjacent bay systems. Spawning salinity has been shown to affect many properties of eggs including the diameter and salinity of neutral buoyancy. Spotted seatrout from two historically different salinity regimes (Matagorda Bay (MB) and Upper Laguna Madre (ULM)) were kept in the laboratory and induced to spawn in three salinities: 20, 30, and 40 ppt. The purpose of this study was to evaluate eggs at each of the three salinities and between the two bay systems. Two-way ANOVA showed a significant effect on the egg diameter of bay and spawning salinity, and a significant interaction between bays and spawning salinity. No significant difference in size at hatch was found between spawning salinities or between bays. Hatch rates in spawning salinity were >90% in all cases. Regression of wet weight on spawning salinity was highly significant for both bays. Eggs spawned in 20 ppt have the largest wet weight and eggs spawned in 40 ppt have the smallest wet weight, irrespective of parental bay origin. Percentage of water varied between 92% for 20 ppt spawned eggs and 86% in 40 ppt spawned eggs. Neutral Buoyancy Salinity (NBS) of eggs increased with increasing spawning salinity. Eggs spawned by the Upper Laguna Madre fish held at 20 ppt were not positively buoyant at 20 ppt. The results of this study suggest that spotted seatrout are locally adapted to the prevailing salinity regime within an estuary.     

Routine metabolism of juvenile spot, Leiostomus xanthums (Lacépède), as a function of temperature, salinity and weight

Routine oxygen consumption rates of juvenile spot, Leiostomus xanthums , were measured over a range of temperatures, salinities and fish weights. As predicted, Q O₂ increased with temperature and decreased with body weight. However, Q O₂ decreased with decreasing salinity and did not show the expected minimum at isosmotic concentrations. The data are best described by the relationship: log₁₀ Q O₂ (mg O₂ g⁻¹ h⁻¹) = 0.129 log_lo salinity (%0) + 1.604 log₁₀ temperature (°C)-0.1401og₁₀(g)-2.767.     

Consumption and gut evacuation rate of laboratory‐reared spotted seatrout (Sciaenidae) larvae and juveniles

The temperature and mass dependence of maximum consumption rate was measured for larval and early juvenile spotted seatrout Cynoscion nebulosus . Maximum consumption ( C _MAX) estimates were obtained from feeding and gut evacuation experiments on larvae (3·8–19 mm standard length, L _S) at three temperatures (24, 28 and 32° C), and maximum consumption experiments on juveniles at three temperatures (20, 26 and 32° C). Feeding levels were determined for larvae fed live prey ( Brachionus plicatilis and Artemia salina ) ad libitum . The midgut and total evacuation times were estimated for fish feeding continuously and discontinuously using alternate meals of tagged and untagged live prey. Temperature and fish size had significant effects on gut evacuation and consumption. The gut evacuation time increased with increasing fish size, and decreased with increasing temperatures. Mass‐specific midgut contents increased for small larvae <0·156 mg dry mass ( M _D)( c . 4 mm L _S), and decreased for larger larvae and juveniles. Maximum consumption was modelled by fitting a polynomial function to a reduced dataset of individuals feeding at high levels. The C _MAX model predicted an initial increase in specific feeding rate from 70 to 155% M _D day⁻¹ for small larvae, before declining for larger larvae and juveniles.     

A dispersal-dependent zone of introgressive hybridization between weakfish, Cynoscion regalis, and sand seatrout, C. arenarius, (Sciaenidae) in the Florida Atlantic

Five diagnostic codominant nuclear DNA markers and a diagnostic mitochondrial DNA marker were used to survey weakfish (Cynoscion regalis) and sand seatrout (C. arenarius), with particular focus on heretofore uncharacterized juvenile populations along the Florida (FL) Atlantic coast. Geographic and reproductive ranges of weakfish and sand seatrout were shown to overlap on the Atlantic coast along north and central FL. An active bidirectional zone of introgressive hybridization exists between these taxa, centered in the St Johns River, FL. Strong patterns of Hardy-Weinberg, linkage, and cytonuclear disequilibrium and a bimodal hybrid index distribution were observed for juvenile cohorts in the zone center, coupled with narrow (～240 km) concordant clines. Parental forms had disparate habitat preferences; hybrid forms occurred predominantly in intermediate habitats. All genetic data were consistent with the hypothesis that the C. arenarius-C. regalis hybrid zone is maintained by a dynamic equilibrium between continued interspecific gene flow and one or more opposing forces. Cytonuclear analyses indicated that parental forms mate assortatively in the zone but that mate recognition was imperfect. Ethological mating dynamics are likely stabilized by some form of endogenous or exogenous postfertilization selection against hybrids such that parental taxa will likely continue to evolve independently.