David A. Bengtson

Aquaculture of summer flounder (Paralichthys dentatus): status of knowledge, current research and future research priorities

Commercial aquaculture of summer flounder began in 1996. Both the research leading to commercialization and the production itself have been heavily oriented toward the hatchery phase. Producers of summer flounder larvae experience many of the same problems that producers of turbot and Japanese flounder larvae have faced and, in some cases, overcome. The extended period of metamorphosis and settling of summer flounder leads to problems with cannibalism and weaning to formulated diets that appear more severe than those in turbot. Producers are experimenting with both recirculation systems and net pens to identify the equipment that optimizes grow-out production. The major research problems that industry and academia should jointly address are (1) establishment of broodstock selection programs, (2) identification of broodstock nutritional requirements, (3) development of methods for volitional spawning by broodstock, (4) management of the microbial ecology in larval rearing tanks, (5) synchronization of metamorphosis to reduce cannibalism and enhance weaning, (6) optimization of production systems and diets for grow-out, and (7) reduction of disease through good husbandry and vaccination.

Growth, survival and size-selective predation mortality of larval and juvenile inland silversides, Menidia beryllina (Pisces; Atherinidae)

A series of laboratory and field experiments were conducted to determine the relative importance of food limitation and predation as sources of mortality for Menidia beryllina (Cope) larvae and juveniles. Seven-day experiments using in situ mesocosms to exclude predators demonstrated significant growth (mean instantaneous growth rate of 0.122–0.135 day−1) and survival (mean 88–89%) for M. beryllina larvae in a Rhode Island, USA, estuary. These results suggest that food was not limiting for growth or survival and, therefore, that predation is likely the primary source of mortality for young-of-the-year (YOY) M. beryllina. Predation experiments were conducted in laboratory aquaria and in in situ mesocosms to assess the size-selectivity of potential predators. Laboratory-reared striped bass, Morone saxatilis Walbaum and field-collected white perch, Morone americana Gmelin, crevalle jack, Caranx hippos Linnaeus and bluefish, Pomatomus saltatrix Linnaeus, were presented with a choice of two or three size classes of laboratory-reared M. beryllina and allowed to feed for 3–24 h. For field-collected predators the experimental prey size range was similar to the size present in the field. Striped bass, white perch and crevalle jack selectively preyed on the smallest size classes. Bluefish, however, selectively preyed on the largest size class. These results suggest that size-specific survival of YOY M. beryllina may vary spatially and temporally depending on the particular suite of predators encountered by individual populations or cohorts. However, in the estuary studied predation mortality appears to be directed towards the larger members of the M. beryllina cohort.

Arachidonic acid requirements in larval summer flounder, Paralichthys dentatus

The arachidonic acid (20:4n-6,AA) requirements of larval summer flounder weredetermined for the rotifer- and Artemia-feeding stages. Experimental emulsionscontained adequate n-3 highly unsaturated fattyacid (HUFA) ratios and emulsion levels of AAwere set at 0, 3, 6, 9, and 12% (AA0, AA3,AA6, AA9, and AA12). Examination of fatty acidlevels in live feeds and larval tissuesconfirmed the physiological incorporation offatty acids relative to dietary levels. In thefirst experiment, survival, growth, andsalinity tolerance (2-h in 70‰) were measuredat 18 days after hatch (dah) after feeding thelarvae the various levels of AA. Larvae fedAA6-enriched rotifers were better able tosurvive the salinity tolerance test. AAenrichment up to 12% had no effect on growthand survival. In the second experiment, larvaewere fed AA0- or AA6-enriched rotifers until 23dah, followed by unenriched 24- and 48-h Artemia nauplii until 32 dah. These larvaethen were subdivided and fed AA-enriched Artemia from 33-45 dah. At the end of thisexperiment, larvae fed AA6-enriched rotifershad higher survival, increased growth, andsurvived better in the salinity tolerance test(2-h in 80‰). The enrichment of Artemiadid not have any effect on these variables.Thus, the provision of AA6-enriched rotifers(10 mg AA g−1 DW) early in larvaldevelopment may serve to enhance larval stresstolerance at the rotifer stage while alsoincreasing larval survival, growth, and stresstolerance later in the Artemia stage.

Reversed Asymmetry in Laboratory-Reared Summer Flounder

Abstract Eggs from laboratory-spawned summer flounder (Paralichthys dentatus) were hatched, and larvae were reared through metamorphosis in captivity. The incidence of juveniles showing a reversed asymmetry under these conditions was high – 4.4% of fish that completed metamorphosis – given that only two cases of reversal in this species have been reported previously. The importance of directionality of ocular asymmetry in flatfishes is addressed.

Effects of photoperiod and temperature on the regulation of the onset of maturation in the estuarine fish Menidia beryllina (Cope) (Atherinidae)

Abstract Menidia beryllina (Cope) is an annual species that inhabits coastal estuaries along the east coast of the United States from Cape Cod to the Gulf of Mexico. In Rhode Island, USA, its peak spawning time and the duration of the spawning vary among years and estuaries. However, the onset of gonadal maturation is more consistent (early May), suggesting that it may be regulated by more consistent cues than those regulating spawning. To determine the effects of photoperiod and temperature on the regulation of the onset of maturation two laboratory experiments were conducted and the results compared to field observations. In the first experiment, fish collected from the field in February were exposed to each of four treatments: increasing photoperiod/increasing temperature; increasing photoperiod/low temperature; low photoperiod/increasing temperature; and low photoperiod/low temperature. Only fish exposed to both increasing photoperiod and increasing temperature were able to complete maturation. Fish exposed to low photoperiod and increasing temperature responded by enlarging their livers, a response that was also observed in field fish collected in the fall. Fish exposed to the remaining two treatments neither matured their gonads nor enlarged their livers. In the second experiment fish collected from the field in early March were exposed to three treatments with different photoperiod regimes (daylight constant at 9.5 h, increasing up to 12 h, or increasing up to 15 h) and one increasing temperature regime. Fish in the 9.5-h treatment initiated maturation but were not able to complete the process, those exposed to the 12-h photoperiod matured and spawned for a short period of time before the gonads began to regress, and those exposed to the 15-h photoperiod matured and spawned large numbers of eggs throughout the remainder of the experiment. The 9.5- and 12-h photoperiod exposures also resulted in accumulation of reserves in the liver in both females and males. The 15-h photoperiod treatment resulted in liver enlargement in females, which were undergoing vitellogenesis, but not in males. Males exposed to the 9.5- and 12-h photoperiod accumulated significantly more visceral fat than those exposed to the 15-h photoperiod. In females, the amount of visceral fat accumulated was inversely proportional to the hours of light. These findings suggest that this species has evolved mechanisms that enable it to anticipate the coming of winter as well as the coming of suitable breeding conditions and ensure that it exhibits the appropriate response at the appropriate time (reserve accumulation for the winter or gonad maturation in the spring).

Metamorphosis in summer flounder: effects of acclimation to low and high salinities

The aim of these studies was to provide larviculturists with information about the effect of low and high salinities on the survival, growth, and development of summer flounder (Paralichthys dentatus) with the goal of enhancing commercial success of land-based flounder culture. Larvae were transferred after first feeding and prior to the onset of metamorphosis and monitored for 3 to 7 weeks. In experiments 1 and 2, larvae were raised in 2-l bowls containing seawater altered to salinities of 14, 30, or 38 ppt at starting densities of 3 premetamorphic and 6 prometamorphic larvae/l, respectively. Survival and development were not affected, nor was growth in experiment 1. Growth was affected in experiment 2, and larvae in 14 ppt had greater total lengths than larvae in 38 ppt. In experiments 3 and 4, larvae were raised in 38-l tanks at starting densities 4 and 3 prometamorphic larvae/l, respectively, containing water with salinities of 8, 30, or 38 ppt or in salinities of 8 and 30 ppt. Again, survival was not significantly affected by salinity. In experiment 3, 8 ppt resulted in better growth and more advanced development; however, no differences were found in experiment 4. Overall, 38 ppt may tend to result in adverse effects. Larval summer flounder are not adversely affected by low salinity (8 or 14 ppt) and were observed in some cases to grow and develop better in environments more closely approximating their internal environment.

Investigations into the causes of early larval mortality in cultured summer flounder (Paralichthys dentatus L.)

Abstract Experiments were conducted to investigate larval mortality in cultured summer flounder during the first 2 weeks after hatch. The influence of feeding success, parentage, addition of algae, water quality, and the microbial community on mortality during this period were investigated. Larvae were raised in 2-l bowls at initial densities of 50 or 75/l with mild aeration, 12 L:12 D photoperiod, and regular 50% water changes. In all experiments, mortalities were recorded and removed daily. In the first two experiments, daily samples of larvae were taken to assess feeding success and to relate that to survival. The second experiment investigated the effects of both feeding success and the addition of algae to larval culture bowls on larval survival. The third experiment investigated the effects of water quality and bacterial load on survival during the experimental period. The first two experiments indicated that failure to establish feeding is probably not the cause of catastrophic mortality of the larvae, although a statistical relationship existed between feeding incidence and survival in two of six cases. High variability (34±38%, n=82) in survival was seen in the first two experiments (both within and between parental crosses) suggesting that catastrophic mortalities were due to rearing conditions in addition to gamete quality. The addition of algae to larval cultures increased survival from 13±24% (n=33) in the absence of algae during the first experiment to 46±39% (n=49) with the addition of algae during the second experiment. The final experiment indicated that larval mortality was not linked to the measured microbial or water quality conditions. The relationship between the percentage of floating eggs at time of fertilization and survival at 10 days after hatch was not significant.

Size-Selective Mortality of Inland Silversides: Evidence from Otolith Microstructure

Abstract Size-selective mortality of age-0 inland silversides Menidia beryllina was assessed by using otolith microstructure analysis and the biological intercept back-calculation procedure. Back-calculated size-frequency distributions were compared with the size-frequency distributions of field collections to determine if and when size-selective mortality occurred. These analyses indicated that in both 1990 and 1992, size-selective mortality occurred and was directed at the larger members of the inland silverside cohort. The timing of size-selective mortality varied between the 2 years. In 1990, size-selective mortality occurred during the juvenile stage, whereas in 1992, size-selective mortality occurred during the late larval and early juvenile stages. These results conflict with the prevailing hypothesis that larger individuals have higher survival probabilities than smaller individuals but support our previous finding that juvenile bluefish Pontatoinus saltatrix prefer to prey on the larger members o...

Marine growth and morphometrics for three populations of Atlantic salmon from eastern Maine, USA

Abstract Significant differences in growth and prespawning body morphology were detected among three stocks of Atlantic salmon reared in a common marine environment. Smolts originating from river-specific broodstock of the Machias, East Machias, and Dennys populations were reared at two marine net-pen facilities for 25 months. Significant differences in stock-specific growth were observed among two stocks at both sites, suggesting a genetic basis for the observed phenotypic variation. There was a significant stock effect to the total measured phenotypic variation based on collected truss network analyses. Linear discriminant function analysis of a truss network of morphometric distances allowed for 73% accuracy of stock classification. A thin-plate spline procedure characterized the Machias body form as having a shortened narrow caudal peduncle region, a compressed body with an elongated trunk, and a deeper head region relative to the other two stocks. Phenotypic variation may be associated with hydrologi...

Green-Water Rearing and Delayed Weaning Improve Growth and Survival of Summer Flounder

Abstract The advent of an aquaculture industry for summer flounder Paralichthys dentatus requires that optimal methods be identified for hatchery production. Two experiments were conducted to test strategies for larval rearing and for weaning newly metamorphosed juveniles from live to artificial diets. Rearing of larvae in “green water” (with algae added) resulted in better survival (76.1 ± 6.5%) from days 5–42 after hatching than did rearing in “clear water” (no algae added; 27.8 ± 13.6%), although no differences in growth were apparent. When fish were weaned from live feed beginning at day 45 versus day 57 by either a “gradual” method (7-d weaning period) or an “immediate” method (no weaning period), better survival and growth were obtained with fish weaned at the later age. For both age-groups, fish weaned by the gradual method exhibited better growth, but not better survival, than those weaned by the immediate method. With these data as examples, commercial hatcheries can conduct cost : benefit analys...