Effects of size, culling and social history on growth of cultured elvers, Anguilla anguilla (L.)

The growth of newly-caught elvers was measured after they were separated into four size categories and induced to feed. All sizes grew equally well, indicating that size differences at capture reflected migration history rather than the presence of genetically poor or good growers. Socially experienced groups of elvers of equivalent size grew, on average, less well due to the presence of socially suppressed fish and fish that did not respond so well to handling or a new social environment.
Replicate groups of rapidly-growing elvers were culled at either 2- or 4-weekly intervals. The large animals removed were replaced by animals of modal size and the growth of selected residual size groups was compared to unculled control groups. Culling tended to increase the instantaneous growth coefficient of remaining small elvers but the added elvers of modal size may have exerted a disproportionate influence on mean growth rates.
The effect of large elvers on the growth of small elvers (and vice versa) was monitored using fish labelled subcutaneously with acrylic paint. Separate populations of elvers of each size group were used as controls. After 82 days in mixed populations, about 36% of the original large fish had been' overtaken' by small category fish.
The results suggested that, following disturbance, the expression of growth in elvers was not necessarily related to size or previous hierarchical position but may be governed by behavioural or physiological responses to handling and changed social environments.     

Otolith length/fish length relationship of leptocephali,elvers, and sub-adult (reared) eelsAnguilla anguilla

Synopsis The otolith length and the total fish length of 9 leptocephali, 29 elvers and 51 sub-adult eels were measured. For the 51 eels a significant correlation between otolith and fish length was found. No similar correlation was found for leptocephali and elvers because of their similar total length. It was found that the growth of the otolith from leptocephali and elvers differs from the growth of herring larvae otoliths.     

Evidence of iteroparity in the widely distributed diadromous fish inanga Galaxias maculatus and potential implications for reproductive output

Gaps in understanding variability among populations of inanga Galaxias maculatus in the timing of reproduction were addressed in southern New Zealand (NZ), where G. maculatus constitutes a declining fishery. Reproductive activity was delayed by 1 month on the west coast compared with the east coast and the west coast spawning season was prolonged into winter. The evidence for post‐spawning survival of some fish was unequivocal from histological studies. These older and larger fish contributed disproportionately to egg production. Estimates of fecundity were considerably lower than those previously calculated for NZ populations. The importance of quality habitats being available during critical life history periods are highlighted. It was apparent that some streams supported fish that were larger and in better condition and that this translated into greatly increased fecundity. Future research should focus on whether this is a legacy of these fish experiencing better pre‐settlement marine habitat as larvae, or higher quality instream habitat enhancing the growth and development of adults.     

Feeding behaviour, swimming activity and boldness explain variation in feed intake and growth of sole (Solea solea) reared in captivity

The major economic constraint for culturing sole (Solea solea) is its slow and variable growth. The objective was to study the relationship between feed intake/efficiency, growth, and (non-) feeding behaviour of sole. Sixteen juveniles with an average (SD) growth of 2.7 (1.9) g/kg^0.8/d were selected on their growth during a 4-week period in which they were housed communally with 84 other fish. Selected fish were housed individually during a second 4-week period to measure individual feed intake, growth, and behaviour. Fish were hand-fed three times a day during the dark phase of the day until apparent satiation. During six different days, behaviour was recorded twice daily during 3 minutes by direct observations. Total swimming activity, frequency of burying and of escapes were recorded. At the beginning and end of the growth period, two sequential behavioural tests were performed: “Novel Environment” and “Light Avoidance”. Fish housed individually still exhibited pronounced variation in feed intake (CV = 23%), growth (CV = 25%) and behavior (CV = 100%). Differences in feed intake account for 79% of the observed individual differences in growth of sole. Fish with higher variation in feed intake between days and between meals within days had significantly a lower total feed intake (r = −0.65 and r = −0.77) and growth. Active fish showed significantly higher feed intake (r = 0.66) and growth (r = 0.58). Boldness during both challenge tests was related to fast growth: (1) fish which reacted with a lower latency time to swim in a novel environment had significantly higher feed intake (r = −0.55) and growth (r = −0.66); (2) fish escaping during the light avoidance test tended to show higher feed intake (P<0.1) and had higher growth (P<0.05). In conclusion, feeding consistency, swimming activity in the tank, and boldness during behavioral tests are related to feed intake and growth of sole in captivity.     

Effects of hatchery rearing on Florida largemouth bass Micropterus floridanus resource allocation and performance under semi‐natural conditions

This study examined the growth, activity, metabolism and post‐release survival of three groups of Florida largemouth bass Micropterus floridanus: wild‐caught fish, hatchery fish reared according to standard practice (hatchery standard) and hatchery fish reared under reduced and unpredictable food provisioning (hatchery manipulated). Hatchery‐standard fish differed from wild‐caught fish in all measured variables, including survival in semi‐natural ponds. Hatchery‐standard and hatchery‐manipulated fish showed higher activity levels, faster growth and lower standard metabolic rates than wild‐caught fish in the hatchery. Fish reared under the manipulated feeding regime showed increased metabolic rates and increased post‐release growth, similar to wild‐caught fish. Their activity levels and post‐release survival, however, remained similar to those of hatchery‐standard fish. Activity was negatively correlated with post‐release survival and failure of the feed manipulation to reduce activity may have contributed to its failure to improve post‐release survival. Activity and post‐release survival may be influenced by characteristics of the rearing environment other than the feeding regime, such as stock density or water flow rates.     

Occurrence of eelAnguilla anguilla larvae west of the European continental shelf, 1971–1977

Synopsis Seven hundred and thirty seven eel larvae were caught in 1971 and in the period 1973–1977 by a modified Isaacs Kidd Midwater Trawl off and on the continental shelf west of France and the British Isles. According to Schmidt (1909) no larvae have been caught on the shelf itself except one glass eel. A decrease in number as well as a decrease of older stages from east to west was observed. Besides the maximum in the length frequency distribution some more smaller peaks occurred. No essential size differences existed between 1975 and 1977, between younger and older developmental stages and between northern and southern larvae. Depth preference during daylight was 300–600 m, at night 35–125 m. Mean density of larvae in the preferred depth layers was nearly 1 per 1000 m³. High densities alternated from year to year with low densities which corresponded to the total catch fluctuations of the elver ascent in the River Ems at Herbrum (West Germany) in the following spring. The possible relationships in mortality between the metamorphosing leptocephalus and immigrating elvers and between the emigrating silver eels and the metamorphosing leptocephalus are discussed.     

Investigating food limitations in wild fisheries: the attendance and growth responses of fish at an anthropogenic feeding station within a temperate estuary

To investigate whether wild fish populations are food limited, this study explored whether the provision of supplementary food had a positive effect on the abundance, condition and growth characteristics of estuarine fish assemblages in New Zealand. Feed (7690 kg) was delivered from an anthropogenic feeding station over a 60-week period to a naturally occurring assemblage of wild fish. Yellow-eyed mullet (Aldrichetta forsteri) of juvenile, sub-adult and newly matured size classes were the dominant species actively foraging at the feeding station throughout its operation, whereas larger piscivorous species visited and foraged from the feed station over the summer period only. Other species were present in the wider area of Nelson Haven, but not at the feed station. No obvious changes in the condition factor of yellow-eyed mullet were observed across the period of monitoring, and no changes in their catchability were detected – although marked seasonal variation in catch rates was observed. Results from tag-recapture data identify that the length-based growth rates of yellow-eyed mullet recaptured near the feed station were higher than those of tagged individuals recaptured at a nearby comparison site, and were higher than the growth rates observed in natural populations of yellow-eyed mullet in the wider region. Shifts in the median size of fish, as observed by acoustic observations, agreed with the tag-recapture data. Although some of the results identified were not amenable to statistical analyses, the attendance of yellow-eyed mullet at the feeding station, in combination with the improved estimates of growth by most of the techniques employed, indicates that yellow-eyed mullet are food limited in their natural environment and that the growth performance of these fish can be positively affected by the increased availability of food.     

Inhibitory effect against pathogenic and spoilage bacteria of Pseudomonas strains isolated from spoiled and fresh fish.

The antibacterial effects of 209 Pseudomonas strains isolated from spoiled iced fish and newly caught fish were assessed by screening target organisms in agar diffusion assays. One-third (67 strains) inhibited the growth of one or several of six target organisms (Escherichia coli, Shewanella putrefaciens, Aeromonas sobria, Pseudomonas fluorescens, Listeria monocytogenes, and Staphylococcus aureus), of which S. aureus and A. sobria were the most sensitive. The inhibitory action was most pronounced among the strains producing siderophores, and the presence of iron eliminated the antibacterial effect of two-thirds of the inhibitory strains. Siderophore-mediated competition for iron may explain the inhibitory activity of these strains. All but nine of the inhibiting strains were found to inhibit the growth of 38 psychrotrophic S. putrefaciens strains isolated from spoiling fish and fish products. Siderophore-containing Pseudomonas culture supernatants inhibited growth of S. putrefaciens, as did the addition of iron chelators (ethylenediamine dihydroxyphenylacetic acid [EDDHA]). In particular, Pseudomonas strains isolated from newly caught and spoiled Nile perch (Lates niloticus) inhibited S. putrefaciens. This suggests that microbial interaction (e.g., competition or antagonism) may influence the selection of a microflora for some chilled food products.     

On the biology and food of small-sized fish from North and Baltic Sea areas. IV. Investigations on an eulittoral mud flat at Sylt Island

The fish fauna of an eulittoral mud flat was investigated at Sylt Island (North Sea) with special regard to its food uptake. During the course of a year the following species were caught:Pomatoschistus microps, Anguilla anguilla (elvers),Zoarces viviparus, andGasterosteus aculeatus. Considering the potentially available food, the most abundant organisms of the benthos were harpacticoids and nematodes, whereas in the phytal layer gastropods and gammarids were dominant. The benthic biomass was found to be greatest in spring, while phytal organisms were most abundant in late summer. The greatest fish density was stated in September 1974 though onlyP. microps was present. The fish biomass was highest in spring when the elvers appeared. The main food ofP. microps was epibenthos; in the diet ofA. anguilla phytal organisms dominated over epibenthos, inG. aculeatus suprabenthic organisms were also present. The most prominent food component by biomass was gammarids in all investigated fish, whereas harpacticoids were only dominant in number. During the course of the year the biomass of ingested food yielded highest values in summer and autumn, but lowest in winter. A very great predatory activity was found in September 1974, which possibly caused a grazing effect on harpacticoids. An estimation of the turnover rate of small-sized fish in this month led to a value of 70 mg dry weight m^?2 · d^?1.     

Distribution, growth, diet and foraging behaviour of the yellow-fin notothen Patagonotothen guntheri (Norman) on the Shag Rocks shelf (Southern Ocean)

The distribution, total length ( L _T) frequency and diet of Patagonotothen guntheri are described from 14 bottom trawl surveys conducted on the Shag Rocks and South Georgia shelves in the austral summers from 1986 to 2006. Patagonotothen guntheri (80–265 mm L _T) were caught on the Shag Rocks shelf from depths of 111 to 470 m, but no specimens were caught on the South Georgia shelf. Multiple cohorts were present during each survey and L _T-frequency analysis of these cohorts suggests that growth was slow (von Bertalanffy K = 0·133). Evidence from stomach contents and acoustic data (2005 and 2006) showed that P. guntheri is primarily a pelagic feeder, migrating from the sea floor towards the surface to feed during daylight. The diet of smaller fish (<140 mm) was dominated by copepods, predominantly Rhincalanus gigas , whilst larger fish principally consumed the pelagic hyperiid amphipod, Themisto gaudichaudii and Antarctic krill Euphausia superba . Some larger fish also took benthic prey.     

Variations in feed intake and growth of Baltic salmon and brown trout exposed to continuous light at constant low temperatures

Interindividual variations in feed intake and growth were studied in Baltic salmon Salmo salar and brown trout S. trutta , held under constant low temperatures of 2, 4 or 6° C and continuous light for 2 months. Rates of feed intake and growth were dependent upon rearing temperature, being lowest at 2° C and highest at 6° C. Further, feed intake and growth were initially low, but increased during the course of the experiment in both species and at all temperatures. These results suggest that acclimatization to the rearing conditions may have required several weeks. The increase in group mean feed intake with time was the result of both an increase in the proportions of fish that fed and an increase in feed intake amongst feeding fish. At the same time as feeding and growth rates increased, interindividual variations in feed intake and growth tended to decrease, suggesting that individual fish were acclimatizing to the new rearing conditions at different rates. Thus, the differences in group mean feed intake and growth rates observed at a given temperature reflected interindividual variations among fish making up the groups. This suggests that group rates of feed intake and growth are not only temperature- dependent, but that they are also highly influenced by variability among fish making up the group.     

Growth and reproductive performance of the Asian catfish Clarias macrocephalus (Gunther) fed artificial diets

Four natural ingredient diets similar in nutrient composition (crude protein = 42–44%; P/E ratio = 115–120 mg/kcal) but different in protein sources, were formulated and fed to hatchery-reared catfish to measure the relative performance of the catfish fed alternative broodstock diets. The control feed was a combination of fish-by-catch and commercial fish pellets. In trial I, growth of the catfish was slow over a 36-week period, but some fish became gravid. Diets 1, 2, and 3 and the control feed were tested in trial II. Growth of fish did not differ significantly (P > 0.05) and female fish in all treatments became gravid. For fish induced to spawn from April to August (1994), hatching rate showed significant differences among treatments (P < 0.05). Relative fecundity, fertilization and hatching rates, and production of 3-day-old larvae were significantly different among fish induced to spawn in November (1994) when another incubation setup was used. Among the diets, diets 2 and 3 best enhanced reproductive performance of the catfish.     

The age, growth and diet of a freshwater population of the flounder, Platichthys flesus (L.), in southern England

Otoliths from 640 flounders from the River Frome, a chalk river in southern England, were used for age and growth studies together with length-frequency data. Mean lengths at ages I–V were 89, 151, 225, 304 and 323 mm respectively; these are higher than have been reported for marine populations. All flounders caught were immature and, on reaching the sea, both sexes probably became sexually mature a year older than did the marine populations. Flounder diets in freshwater comprised largely aquatic insects, Mollusca and Crustacea but there were differences between large and small fish; seasonal changes in diet largely reflected prey availability. It is suggested that competition for food and space in estuaries between 0 group flounders have given rise to the migration into fresh water.     

Otolith microstructure and daily age of Anguilla japonica, Temminck & Schlegel elvers from the estuaries of Taiwan with reference to unit stock and larval migration

To test the hypothesis that the Japanese eel, Anguillu japonica , elvers in the eastern and western coasts of Taiwan are recruited from two different spawning grounds and to increase the knowledge of the early life history of the eel, the otolith microstructure and daily age of elvers collected from five estuaries in the coast of Taiwan during December 1989 through February 1990 were examined by scanning electron microscopy. The total length of elvers at arrival in the estuaries was similar among estuaries, averaging c . 56.0 mm and showed a seasonal decrease. The maximum radius of the sagittal otolith of elvers ranged from 124.46 to 181.82μm with a mean of 143.15 ± 12.72 μm. The otolith from centre to edge included an organic-rich primordium (9.20 ± 2.02 μm in diameter), a diffusively calcified core (20.94 ± 1.99 μm), and the daily growth increments; these three layers were probably deposited during the embryonic, yolk sac and feeding period respectively. The growth rate of the otolith was higher at the beginning of early life (0.5−1.0 μm day ⁻¹), lowest at approximately 100-days old (<0.5 μm day⁻¹), and highest 1 month before arrival at the estuary (> 1.0 pm day⁻¹). The mean age for elvers arriving in the estuaries of the coasts of Taiwan was approximately 170.4 ± 21.02 days. Neither the growth pattern of the otolith nor the age of elvers arriving in the estuary were significantly different among estuaries, indicating that the elvers in both eastern and western Taiwan were probably recruited from the same spawning ground. The growth pattern of the otolith in relation to larval migration was analysed.     

Trout in Llyn Alaw, Anglesey, North Wales. II. Growth

The annual and seasonal growth, length/weight relationship, changes in seasonal condition and specific growth rates of 17 types of trout in the newly flooded reservoir, Llyn Alaw, in Anglesey, North Wales, were investigated. Scales and length-weight data from 2076 trout caught by angling and netting during the first 4 years after impoundment were analysed to show the age, origins and growth of all fish both before and after flooding.     

Variance in rates of growth and development in larval and metamorphosing summer flounder, Paralichthys dentatus

Summer flounder, Paralichthys dentatus L., show variance in total length as they progress through larval development and metamorphosis, both in commercial aquaculture hatcheries and in research facilities. This variance leads to serious problems with cannibalism after settlement, if the fish are not ‘graded’ (i.e. separated by size), a labor‐intensive process. In order to document the magnitude of this variation and to try to understand the basis for it, we conducted a series of experiments during (a) the larval period and (b) the process of metamorphosis. The larval experiments were done on fish reared individually in small bowls and repeatedly measured during their development. The metamorphosis experiments were done on fish reared either communally or individually and repeatedly examined to determine the relationships between age, growth and stage of development. There was no relationship between size of larvae at 8 or 9 days after hatch (DAH) and their subsequent size at 30 DAH. During two larval trials, significant events occurred around 20–22 DAH: increased variability in size among offspring from one set of parents, and increased growth without increased variability among offspring from another set of parents. These findings suggest that selective breeding of this species might be a way of reducing size variability. During the metamorphosis trials, we found that rates of development and absolute growth in summer flounder are positively correlated.     

Salinities, not diets, affect strontium/calcium ratios in otoliths of Anguilla japonica

Although otolith Strontium (Sr)/calcium (Ca) ratios have been widely used to reconstruct the past salinity environmental history of anguillid eels, factors affecting the Sr/Ca ratios in otoliths are incompletely understood. Japanese Eel (Anguilla japonica) elvers (mean length 54.7 ± 2.1 mm) were collected in the estuary during their upstream migration and reared at 5 different salinities (0, 5, 15, 25, and 35 psu) and 3 types of feeding conditions (formulated feed, tubifex, and starvation) for 30 days to evaluate the effects of salinity and diets on otolith Sr/Ca ratios. Ca and Sr concentrations in the ambient water significantly increased with salinity (SAL) as [Ca] _water = 15.50SAL − 5.56, and [Sr] _water = 0.21SAL + 0.03, respectively. Sr/Ca ratios in otoliths increased with salinity (SAL) of the rearing water as [(Sr/Ca) × 1000] _otolith = 0.091SAL + 3.790. In diets, Sr/Ca ratios were 4 times higher in tubifex than in formulated feed. However, in otoliths, ANOVA indicated that Sr/Ca ratios did not differ significantly between groups fed on tubifex or formulated feed (p = 0.118). Otolith Sr/Ca ratios were negatively correlated with fish growth rates while the growth rates differed significantly among rearing conditions with different salinities and diets. Partition coefficients of the Sr/Ca ratios from ambient water to fish tissues and otoliths significantly increased with salinity. The Sr/Ca ratios of Japanese Eel otoliths thus were positively correlated with the ambient salinity and decreased with increasing fish growth rate, but was not affected by fish diet.     

Timing is everything: Fishing‐season placement may represent the most important angling‐induced evolutionary pressure on Atlantic salmon populations

Fisheries‐induced evolution can change the trajectory of wild fish populations by selectively targeting certain phenotypes. For important fish species like Atlantic salmon, this could have large implications for their conservation and management. Most salmon rivers are managed by specifying an angling season of predetermined length based on population demography, which is typically established from catch statistics. Given the circularity of using catch statistics to estimate demographic parameters, it may be difficult to quantify the selective nature of angling and its evolutionary impact. In the River Etne in Norway, a recently installed trap permits daily sampling of fish entering the river, some of which are subsequently captured by anglers upstream. Here, we used 31 microsatellites to establish an individual DNA profile for salmon entering the trap, and for many of those subsequently captured by anglers. These data permitted us to investigate time of rod capture relative to river entry, potential body size‐selective harvest, and environmental variables associated with river entry. Larger, older fish entered the river earlier than smaller, younger fish of both sexes, and larger, older females were more abundant than males and vice versa. There was good agreement between the sizes of fish harvested by angling, and the size distribution of the population sampled on the trap. These results demonstrate that at least in this river, and with the current timing of the season, the angling catch reflects the population's demographics and there is no evidence of size‐selective harvest. We also demonstrated that the probability of being caught by angling declines quickly after river entry. Collectively, these data indicate that that the timing of the fishing season, in relation to the upstream migration patterns of the different demographics of the population, likely represents the most significant directional evolutionary force imposed by angling.     

Environmental correlates of life-history variation in juvenile chinook salmon, Oncorhynchus tshawytscha (Walbaum)

Throughout its native North Pacific, the chinook salmon, Oncorhynchus tshawytscha (Walbaum), exists as twolife-history types that aredistinguished by the age at which juvenile salmon migrate to sea as smolts. 'Stream-type' chinook migrate seaward after I or more years of feeding in fresh water, whereas 'ocean-type' fish migrate to sea as newly emerged fry or after 2–3 months in fresh water. Stream-type chinook predominate in populations distant from the sea south of 56° N, and in both inland and coastal populations north of this point. By contrast, ocean-type chinook predominate in coastal populations south of 56° N, but are rare in populations in more northerly latitudes. Stream-type populations are associated with areas of low 'growth opportunity' (as measured by temperature and photoperiod regimes) and/or areas distant from the sea compared to ocean-type. Geographic variability in juvenile life history is suggested to result, in part, from environmental modulation of smolting timing via differences in growth opportunity among geo-graphic areas. In addition, differences in migration distance and temperature regime may result in selection for different sizes at migration among populations which, through differences in growth opportunity, might promote geographic variability in age at seaward migration.     

Interindividual variations in feeding and growth in rainbow trout during restricted feeding and in a subsequent period of compensatory growth

Compensatory growth responses of rainbow trout Oncorhynchus mykiss were studied by examining food intake and growth of individual fish held within groups that were switched between regimes that involved full and restricted feeding. Restricted feeding led to marked interindividual variability in food intake, probably as a result of the establishment of feeding hierarchies. This disparity in food acquisition was reflected in highly heterogeneous growth amongst the fish fed low rations. When fish were transferred from restricted to full rations, they became hyperphagic and displayed high rates of growth. Growth compensation was most marked amongst those fish which had shown the poorest growth during the period of feed restriction. These results suggest that the feeding hierarchies established under feed restriction did not persist, but were rapidly broken down when food became increasingly available, enabling the previously suppressed fish to gain access to food and to display rapid growth.