Individual Fish Weight Research Articles

Reproductive biology and some observation on the age, growth, and management of Tilapia zilli (Gerv, 1848) from Lake Timsah, Egypt

In the present investigation, some biological aspects of Tilapia zillii from Lake Timsah were studied. This was carried out on 700 fish samples collected during the period from October, 2002 to September, 2003. Total length of females ranged from 7 to 16 cm and males from 8 to 21 cm. Age determination based on length frequency distributions showed that the life span are two and four years for females and males, respectively. Estimates of growth parameters are widely used for detailed assessment and modeling of fish species and fisheries. The length-weight relationship, age at length zero (to), growth coefficient (K), asymptotic length (L¥) and rates of natural (M) and total mortality (Z) were estimated for T. zillii from Lake Timsah. Length-weight relationships were calculated from individual fish length and weight records collected from sampling using gill nets. Analyses were obtained by sampling direct from the fisher’s catches. Data were analyzed using the FISAT software. T. zillii, the estimate of K was 0.320 and 0.680 year-1 for males and females, respectively, whilst L¥ i€ was approximately 22.05 and 17.85 cm, for males and females, respectively. T. zillii (males) had the lowest value of K and the largest L¥ i€ than females. Natural mortality was the lowest for T. zillii (females) (M=0.464 and 0.38 year-1) and fishing mortality was highest for T. zillii (males) (F=2.656, 1.88 year-1). In addition, exploitation rate was estimated at E=0.83 and length at first capture (Lc) was calculated at 11.30 cm for males while E=0.78 and (Lc) was estimated as 6.68 cm for females. The majority of fish caught of the main commercial species in the Lake Timsah fishery were from the first age group, at a size far below the optimal length at capture. This suggests growth overfishing for T. zillii in Lake Timsah. Key words: Lake Timsah, Tilapia zillii, age and growth, mortality, gonadosomatic index, length at first maturity, population structure.

Golden Shiner Egg Production during a Spawning Season

AbstractThis study determined the number of eggs produced daily by golden shiners Notemigonus crysoleucas during a 111‐d spawning season and the relationship of egg production to water temperature. On March 15, 2000, four 5.9‐m2, plastic‐lined pools were each stocked with fifty 1‐year‐old golden shiners. Mean (±SD) individual weight of fish at stocking was 9.2 ± 3.7 g. During March 16 through July 4, the spawning substrate in each pool was collected daily and replaced with clean substrate. Eggs were removed from mats in a 1.5% sodium sulfite bath, and the total volume of eggs from each pool was recorded. The mean number of eggs per unit volume (based on twenty‐seven 2‐mL samples) was 704 ± 109 eggs/mL. Approximately 2.75 million eggs were produced by no more than 200 brooders (∼2.4‐kg total mass) over 111 d. Egg production was greatest during the month of May (average = 14,339 eggs·kg−1·d−1). The number of eggs collected daily showed a significant positive correlation with water temperature (P < 0.05) up to 22°C and then declined with further increases in water temperature.

A prospective matched nested case–control study of bacterial gill disease outbreaks in Ontario, Canada government salmonid hatcheries

Early-rearing salmonids in Ontario, Canada government fish hatcheries have been persistently affected by bacterial gill disease (BGD), and outbreaks at these locations have often been associated with high morbidity and mortality. The causative agent of BGD, Flavobacterium branchiophilum, is ubiquitous in fresh water, and outbreaks of BGD are considered to be associated with deleterious environmental conditions. This paper summarizes a 14-month rearing unit-level prospective nested matched case–control investigation at six Ontario government hatcheries (raising a total of six different salmonid species) to identify, and quantify the effects of, important predictors of BGD outbreaks. Ongoing husbandry data were collected on all early-rearing (<9 months of age) fish tank-lots (“tank-lot”=a group of fish from a specific lot existing in a single hatchery tank for a given period during the study time frame) at participating hatcheries, and all outbreaks of BGD were confirmed by light microscopy during the study period. Control tank-lots were selected at the end of the study and matched to individual cases based on time, hatchery, and species. Data were analyzed using logistic regression modeling, controlling for fish age. The final multivariable model indicated that affected tank-lots were significantly more likely to have had lower fish numbers, lower individual fish weights, higher mortality levels and higher feeding rates during the week preceding observed BGD outbreaks than were asymptomatic control tank-lots. Refinements in the observation and manipulation of these factors could therefore aid in the prevention of fish losses associated with observable BGD outbreaks. The predictive (as opposed to causal) nature of the identified factors needs to be considered, and further research is required to understand the relationships between these factors and BGD.

Effects of zooplankton refuge on the growth of tilapia (Oreochromis niloticus) and plankton dynamics in pond

Effects of zooplankton refuge on the growth of tilapia (Oreochromis niloticus) and plankton dynamics in pond were assessed by using 12 land-based experimental enclosures (7 m× 4 m) in a saline–alkaline pond. There were 4 treatments resulting from a combination of 2 stocking densities of tilapia (5 and 10 kg per enclosure) and 2 enclosure types (refuge and nonrefuge). Each treatment had three replicates. In the enclosures with refuge, all fish were excluded from a water column ¼ of the enclosure’s size with a polyethylene net (1.5 cm mesh size). Our field experiment showed that the individual gain in weight of tilapia increased 6.4 times with refuge treatment at the stocking density of 5 kg per enclosure than that in nonrefuge treatment. But at the stocking density of 10 kg per enclosure, the individual fish weight in all enclosures decreased due to the excessive density of fish stocking. Our results also showed that the refuge significantly increased the rotifer biomass and phytoplankton diversity and decreased the phytoplankton biomass (P < 0.05), but the copepod biomass was not affected by the refuge.

Rearing unit-level factors associated with bacterial gill disease treatment in two Ontario, Canada government salmonid hatcheries

Early-rearing salmonids in Ontario Ministry of Natural Resources (OMNR) fish hatcheries have been consistently affected by bacterial gill disease (BGD) (causative agent: Flavobacterium branchiophilum) for many years. Separate retrospective epidemiological investigations of BGD treatments at two OMNR fish hatcheries (Hatcheries A and B) for the 1999 production year were conducted using on-site hatchery records. Both investigations were carried out at the rearing unit-level, with early-rearing (<9 months of age) “tank-lot” as the unit of analysis to identify unique fish populations over time. Multivariable repeated measures logistic regression models were created for both hatchery datasets, controlling for lot-level and species effects. For Hatchery A, the species brook trout (Salvelinus fontinalis) and brown trout (Salmo trutta) were significantly associated with BGD treatment, as well as lower water exchange rate, and higher feeding and mortality percentages during the 2 weeks previous to BGD treatment. At Hatchery B, the species brook trout (S. fontinalis) and splake (Salvelinus namaycush×S. fontinalis) were significantly associated with BGD treatment, as well as lower individual fish weights and treatment for BGD during the previous week. These results emphasize the importance of water quality, feeding rate, fish size and prior mortality on the development of BGD. Significant hatchery and species effects were evident, and future observational research on BGD must account for these factors in their design and analysis.

Evaluation of Poultry By‐product Meal in Commercial Diets for Hybrid Striped Bass, Morone chrysops ♀ × Morone saxatilis ♂, in Pond Production

Abstract The efficacy of replacing fish meal with petfood‐grade poultry by‐product meal (PBM) on an ideal protein basis in commercial diets for hybrid striped bass (HSB) was evaluated under production conditions in pond culture. A generic production diet (GEN) for HSB was formulated to contain 45% protein, 12% lipid, and 3.7 kcal/kg. Protein in the generic diet was supplied by a mix of animal and plant sources typically used by the industry that included more than 20% select menhaden fish meal and less than 10% PBM. A positive control diet (GEN + AA) was formulated by supplementing the generic diet with feed‐grade Met and Lys to match the level of those amino acids in HSB muscle at 40% digestible protein. Substitution diets were formulated by replacing 35, 70, or 100% of fish meal in the GEN diet with PBM on a digestible protein basis and then supplementing with Met and Lys (designated 35PBM, 70PBM, and 100PBM, respectively) as needed to maintain concentrations equal to those in the GEN + AA diet. Diet formulation and extrusion were conducted by a commercial mill, and all diets met or exceeded known nutritional requirements for HSB. Twenty 0.10‐ha ponds (4 ponds/diet) were randomly stocked with juvenile HSB (76 ± 10 g; mean ± SD) at a density of 7400/ha and fed for 600 d (October 2004 to May 2006). Diets were fed once daily to apparent satiation to a maximum of 95 kg feed/ha. Total weight and number of fish in each pond were determined at harvest. Weight distributions in each pond were estimated by selecting every 15th fish during harvest. Subsets of ten fish from each of these samples were selected randomly for the determination of body composition and nutrient and energy retention. The availability of indispensable amino acids as well as ammonia production from the commercial test diets were determined in separate tank trials. Most production characteristics were not statistically different (P &gt; 0.10) among dietary treatments. Distributions of individual fish weights from each of the ponds were not affected by poultry by‐product level in the diet. Multivariate analysis of body compositional indices grouped diets into two clusters composed of GEN, GEN + AA, 35PBM vs. 70PBM, and 100PBM mainly because fish fed the 70PBM and 100PBM diets had greater (P = 0.001) body fat (visceral somatic indices) than fish fed the other diets. Ammonia production in tanks was not different among diets and peaked 6–8 h after feeding when fish were fed at 1.5% of body weight; ammonia‐N excretion ranged from 197 to 212 mg/kg/d and 18.5–21.5% of nitrogen intake. Some imbalances in the levels and ratios of selected amino acids to Lys were found in the diets containing higher amounts of PBM and were attributed to a lack of accurate availability coefficients during formulation for some dietary proteins. These imbalances in essential amino acids may have been the predominant factor in the somewhat fattier fish observed fed diets containing the two highest levels of PBM. Nevertheless, these results from fish stocked at commercial densities and raised to market size in ponds suggest that formulating diets on an available amino acid basis for all protein sources while balancing limiting amino acids, particularly Met, Lys, Thr, and Trp, on an ideal protein basis will yield significant improvements in HSB performance when fed commercial diets in which all fish meal is replaced with PBM.

Long-term growth and movement in relation to food supply and social status in a stream fish

Long-term patterns of residency, growth, and movement among drift-feeding fish in relation to their social status and macroinvertebrate drift were determined. Each month, over 21 months, we quantified macroinvertebrate drift, fish abundance, and biomass, recorded individual fish lengths, weights, and social rank, and monitored individual diel activity of fish residing in different pools. Despite marked variation in food supply between pools, average growth rates within each pool were not significantly different. However, size and growth of the largest (dominant) fish (i.e., ones that had priority access to preferred feeding positions and times) was positively related to food supply. In contrast, the distribution and growth of smaller subordinate fish were only weakly related to the distribution of resources. Over the 21 months of study, growth rates of the first-ranked fish were high and they rarely relocated to a new pool. Second- and third-ranked fish exhibited comparatively low growth rates and relocated to new pools more frequently than fish of any other ranks (higher or lower). Movement of these fish from their original pool of residency did not always result in increased growth rates. Overall, our study demonstrates that short-term behavioral responses to spatial and temporal resource availability and intraspecific competition (i.e., social status) interact to determine long-term patterns of distribution, movement, and growth.

Pulp and paper mill effluent treatments have differential endocrine‐disrupting effects on rainbow trout

Endocrine disruption (ED) effects due to pulp and paper mill effluents extracts involving different industrial procedures and effluent treatments (nontreated, primary, and secondary treated) were evaluated using immature triploid rainbow trout in a pulse-exposure toxicity experiment. The protocol involved the use of intraperitoneal injection of mill extracts (solid-phase extraction [SPE]) corrected for individual fish weight and included several laboratory standards (steroidal hormones and phytosterols). Biological endpoints at two different levels of biological organization were analyzed (molecular and individual organism). Results indicated that nonsignificant changes were observed in the individual physiological indices represented by condition factor, liver somatic index, and gonad somatic index during the experiment. Significant induction of liver ethoxyresorufin-O-deethylase activity was observed between different effluent treatments and experimental controls. Significant endocrine-disrupting effects at the reproductive level were observed in all effluent treatments involving significant increments in plasma vitellogenin (VTG) levels. Fish exposed to untreated effluent extracts had significantly higher VTG levels compared to fish exposed to primary and secondary treatment effluent extracts, indicating a decrease of the estrogenic effect due to the effluent treatment. The present study has shown that for the Chilean pulp and paper mill SPE extracts evaluated, an endocrine disruption effect was induced in immature triploid rainbow, reaffirming the significant estrogenic effects demonstrated previously in laboratory and field experiments.

Length-Weight Relationships of Select Common Nearshore Southern California Marine Fishes

Growth dynamics of marine fish is often critical for fisheries analysis. Frequently only lengths are measured during ecological surveys, occasionally with aggregate weights by species (Allen and Herbinson 1990; Pondella and Allen 2000; Allen et al. 2002). This information often allows for estimation of age-class composition through length frequency analysis, but can mask the importance of individual fish weights to the overall community structure, such as the inclusion of one, or a few, large individuals among a predominantly small catch. Over time, authors have reported length-to-weight relationships for specific species in the course of specific life history investigations, but these studies are rarely conducted on forage species. Cailliet et al. (2000) compiled many of the relationships that were available at the time, but numerous species still lack such basal information. Their database, however, contained 124 species, many of which did not have length-weight relationships available. Love et al. (2002) also compiled additional information on several rockfish species (Sebastes spp) and Sebastolobus alascanus. Since 1979, impingement surveys at coastal generating stations routinely recorded the length of nearly all impinged fish during a given survey. Most of these surveys, however, recorded the aggregate weight by species, a protocol consistent with most ecological studies in southern California. Specific, focused studies were occasionally undertaken during which the individual length and weight of a subset of individuals were recorded. Moreover, instances when only a single individual of a specific species was impinged, a defacto length-weight relationship data point was recorded. During these studies the appropriate length; standard (SL), total (TL), or disc width (DW) was measured to the nearest millimeter (mm) and weight recorded to the nearest gram (g). Data from impingement records 2001–2006, recorded at generating stations from San Diego County to Los Angeles County, California were reviewed to generate speciesspecific length-weight relationships (Tables 1 and 2). A total of 59 species were identified with length and weight recorded for greater than 10 individuals. These included both 54 ray-finned fishes (Class Actinopterygii) and 5 elasmobranch species (Subclass Elasmobranchii). All were common to the Southern California Bight (Miller and Lea 1972; Love et al. 2005). Forty-two of these species were included in the Cailliet et al. (2000) species list, but only 27 had a length-weight relationship listed. The length-weight relationships of fishes typically fit the non-linear equation W 5 aL, where W 5 weight (g), L 5 length (mm), and a and b are derived constants. A best fit line was plotted for each distribution using MS Excel. The determination of the best fit was based on the R-value. Fifty-six species were best described by the traditional non-linear equation W 5 aL. Two species, Anchoa compressa and A. delicatissima were best described by a linear relationship and one species, Seriphus politus, was best described by an exponential function. The minimum and maximum length (mm) recorded for each

Production Responses of Channel Catfish to Minimum Daily Dissolved Oxygen Concentrations in Earthen Ponds

AbstractThis study determined the effects of the minimum daily dissolved oxygen (DO) concentration on the production parameters of channel catfish Ictalurus punctatus in earthen ponds. Fifteen 1‐acre ponds (five ponds per treatment) were managed as high‐oxygen (minimum DO concentrations averaging 4.37 ppm or 54% air saturation from June through September), medium‐oxygen (minimum DO concentrations averaging 2.68 ppm or 33.2% air saturation), or low‐oxygen treatments (minimum DO concentrations averaging 2.32 ppm or 28.7% air saturation) using one 5‐hp electric paddlewheel aerator per pond. Fish in the high‐, medium‐, and low‐oxygen treatment ponds were fed a mean total of 14,008, 13,212, and 12,607 lb/acre of 28%‐protein floating feed, respectively. Net production paralleled the total amount of feed fed, averaging 5,772, 5,278, and 5,113 lb/acre in the high‐, medium‐, and low‐oxygen treatments, respectively. Individual fish weight at harvest also showed a similar trend, averaging 1.37, 1.33, and 1.30 lb in the high‐, medium‐, and low‐oxygen treatments, respectively. No visible stress responses were observed in any ponds during this study. Total aeration averaged 5,245, 2,518, and 1,337 hp‐h/acre in the high‐, medium‐, and low‐oxygen treatments, respectively. Treatments with higher minimum DO concentrations had significantly higher nitrite‐nitrogen, suspended solids, chlorophyll a, and pH and lower Secchi disk visibility, alkalinity, and hardness; however, no water quality parameters exceeded the normal acceptable range for channel catfish. While the cost of electricity must be considered, maintaining a minimum daily DO concentration of 2.3–2.5 ppm is suggested as a compromise between maximizing both the amount of feed fed and fish production while minimizing aeration costs.

Replacement of Fish Meal with Poultry By‐product Meal as a Protein Source in Pond‐raised Sunshine Bass, Morone chrysops ♀ × M. saxatlis ♂, Diets

Abstract Replacement of fish meal (FM) as a protein source with alternative sources of protein in aquaculture diets has been widely explored in aquaculture. The goal of replacement of FM in production diets is to maintain growth, lower production costs, and increase sustainability. Evaluation of the replacement of FM with poultry by‐product meal (PBM) in phase II sunshine bass diets, Morone chrysops × M. saxatilis, was conducted in ponds over 246 d. Four diets were formulated to be isonitrogenous (37%) and isocaloric (4 kcal/g) with different levels of FM replacement with PBM (0, 33, 67, and 100%, Diets 1–4, respectively). Twelve ponds were stocked with 400 phase II sunshine bass (mean weight 5.6 g) and randomly assigned one of the four diets. Fish were fed below satiation based on predicted growth and feed conversion, initially once daily (1700 h) and then twice daily (0700 and 1700 h) as water temperatures and feeding activity increased. Diets were evaluated based on production and performance indicators, body composition, and economic analysis. Production results revealed no significant differences in mean final individual fish weight (511 ± 21 g), net production (4257 ± 247 kg/ha), and survival (85 ± 2%). No significant differences occurred between the performance indicators: mean feed conversion ratio (2.47 ± 0.11), specific growth rate (1.84 ± 0.02), and protein conversion efficiency (23 ± 1.3%). Body composition was statistically similar for mean percent fillet weight (49 ± 0.6%) and percent intraperitoneal fat (9.8 ± 1.0%); however, the hepatosomatic index was significantly different between Diets 3 (3.7 ± 0.1%) and 4 (3.2 ± 0.1%). Mean proximate analysis of whole fish (dry weight basis) was not significantly different among treatments yielding the following: percent protein (46 ± 0.4%), lipid (47 ± 1.3%), and ash (8 ± 0.7%). Mean fillet composition (dry weight basis) also revealed no significant differences: percent protein (72 ± 0.8%), percent lipid (30 ± 1.6%), and percent ash (5 ± 0.2%). Proximate analysis was also performed on the diets and revealed a significantly lower protein content in Diet 3 (34.3 ± 0.5%) compared to the other diets (37.1 ± 0.4%). Amino acid analysis of the diets indicated a possible deficiency in methionine in Diets 3 and 4. Based on production, performance, and body composition, the results indicate that complete replacement of FM with PBM in sunshine bass diets is feasible; however, economic analysis suggests that the replacement of FM with PBM may result in reduced revenue over feed costs.

Production and Associated Economics of Channel Catfish Fed Different Practical Diets in the Fingerling–Stocker Phase of Culture

AbstractTwo practical diets that differed in cost based upon protein sources were evaluated as treatments relative to production and associated economics of the fingerling–stocker phase of culture for channel catfish Ictalurus punctatus. The commercially manufactured fingerling feeds contained approximately 35% crude protein and were fed to fingerling channel catfish (mean weight = 13.8 g; mean length = 12.2 cm, respectively) stocked into experimental earthen ponds (0.05 ha) at a density of 123,550 fish/ha. There were five replicates (ponds) per treatment. The fish were fed daily to satiation, unless some physical or biological factor warranted an abstention of one or more days. After 180 d, all ponds were harvested by seine and then completely drained. Mean individual harvest weight and length of fish fed the high‐cost practical diet were significantly greater than those of fish fed the low‐cost diet. Survival and feed conversion ratios were not significantly different between dietary treatment groups. The percentage of harvested fish exceeding 114 g, a size that escapes predation by cormorants Phalacrocorax spp., ranged from 47% to 56% for the low‐cost treatment and from 72% to 77% for the high‐cost treatment. Individual stocker production cost was US.217 for the low‐cost dietary treatment and .258 for the high‐cost treatment. Economic analyses were used to compare the three‐phase (modular) production scheme (which incorporated the two diet‐dependent fingerling–stocker management scenarios) with the traditional multiple‐batch system (partial harvest and topping off). Total production costs for the three‐phase system were .04–0.24 per kilogram less than those of the multiple‐batch production system. Total cost of food fish production in the three‐phase system ranged from $1.406 (low‐cost diet) to $1.417 (high‐cost diet) per kilogram. The fingerling–stocker phase offers a variety of management options to meet different production goals, and the three‐phase system appears to be a cost‐effective alternative management strategy for the farming of channel catfish.

Effects of an Extended Hatchery Phase and Vaccination against Enteric Septicemia of Catfish on the Production of Channel Catfish, Ictalurus punctatus, Fingerlings

Abstract.—The present study was conducted to evaluate production management methods to improve overall survival of channel catfish, Ictalurus punctatus, fry to the fingerling stage by incorporating the use of a live, attenuated vaccine against Edwardsiella ictaluri and employing an extended hatchery phase. In this experiment, four treatments were used. In Treatment 1, 10‐d posthatch (PH) fry were vaccinated and then directly stocked into earthen ponds. In Treatments 2 and 3, 10‐d PH fry were sham‐vaccinated (control) and vaccinated, respectively, kept in nursery tanks for 22 d, and then stocked into earthen ponds. Fry in Treatment 4 were sham‐vaccinated at 10 d PH, kept in nursery tanks for 22 d, and then vaccinated prior to stocking into earthen ponds. Mean fingerling yield at harvest ranged from 4716 kg/ha in Treatment 1 to 8112 kg/ha in Treatment 4. Mean individual fish weight ranged from 38.8 g in Treatment 1 to 40.9 g in Treatment 4, and feed conversion ratios (FCR) ranged from 1.15 in Treatment 4 to 1.51 in Treatment 1. Mean survival ranged from 47.5% in Treatment 1 to 73.4% in Treatment 4. In specific comparisons to evaluate the nursery effect (Treatments 1 and 3), yield and overall survival were significantly different (P &lt; 0.05) between these two treatments. In specific comparisons to evaluate the effect of the use of the vaccine (Treatments 2, 3, and 4), overall survival was significantly different (P &lt; 0.05) between Treatment 2 (sham‐vaccinated control with nursery phase) and Treatment 4 (vaccinated at 32 d PH with nursery phase). No significant differences (P &gt; 0.05) in yield, average weight, and FCR were observed between treatments. Results indicate that implementing an extended hatchery phase and vaccination strategy with older fry can improve overall survival of fingerling fish.

Length-weight relations and growth rates of dominant fishes of the Salton Sea: implications for predation by fish-eating birds

The Salton Sea is the largest lake in California. Inflows are primarily from agricultural runoff, which render it eutrophic and able to support extensive fisheries. The lake and its surrounding wetlands are critical links for the Pacific Flyway, providing refuge and food for an extensive and diverse avifauna. We document fish size distributions, body shapes and growth rates and consider how they determine availability of fish to birds. Tilapia (Oreochromis mossambicus x O. urolepis hornorum) is the most important resource for fish-eating birds. Tilapia grow fast to a size that is readily handled by large birds and are the most abundant fish in the lake. Bairdiella (Bairdiella icistia) grow to a smaller size over a longer period. Corvina (Cynoscion xanthulus), sargo (Anisotremus davidsoni), and shad (Dorosoma cepedianum) are less important for fish-eating birds because they quickly grow to sizes outside bird handling capabilities (corvina) or are not readily catchable and currently of low abundance (shad and sargo). Shape, in particular girth, determines the weight of the largest individual fish that a bird can handle; a slender 1,000 g corvina, for example, being more easily ingested than a deeper-bodied 1,000 g tilapia. Shape, however, is of secondary importance to growth rate in determining importance of a fish species in a bird's diet. Predation by corvina on tilapia and other smaller species may have historically played a large role in determining availability of fish to fish-eating birds.

Epidemiology of blindness and cataracts in cage reared ongrown Atlantic halibut Hippoglossus hippoglossus

Cataract pathology, intensity and severity, and temporal development were investigated in halibut in the unusual location for halibut of solely the anterior lens. This manifested as an opacity and there was fibre disruption in the anterior cortex with vacuoles. Individual eye blindness was 17–26% and significantly higher in non-migrated eyes, but bilateral blindness was only 0–4%. Cataract prevalence varied greatly in fish between sea pens, from 29 to 83%, and there was a significant increase on time, with a higher prevalence in migrated eyes. The cataract severity score per individual eye increased from 1.24 to 1.69 (score range 0–3) between sampling dates but there was no significant difference in severity in migrated and non-migrated eyes. Neither cataract prevalence nor severity were related to individual fish weight suggesting that growth per se was not an important influence. Physical damage largely affected the protruding non-migrated eye as a consequence of aggression in the hatchery and consequently physical damage of halibut in sea cages was well healed. Damage from UV light and nutritional deficiencies were regarded as possible factors in the formation of these anterior cataracts in ongrown halibut.

Effect of Multiple‐batch Channel Catfish, Ictalurus punctatus, Stocking Density and Feeding Rate on Water Quality, Production Characteristics, and Costs

Abstract To quantitatively define relationships among stocking densities, feeding rates, water quality, and production costs for channel catfish, Ictalurus punctatus, grown in multiple‐batch systems, twelve 0.1‐ha earthen ponds were stocked at 8,600, 17,300, 26,000, or 34,600 fingerlings/ha along with 2,268 kg/ha of carryover fish. Fish in all ponds were fed daily to apparent satiation using 32% protein floating feed. Temperature and dissolved oxygen in each pond were monitored twice daily; pH weekly; nitrite‐N, total ammonia nitrogen, and Secchi disk visibility every 2 wk; nitrate‐N, chlorophyll a, total nitrogen, total phosphorus, and chemical oxygen demand monthly; and chloride every other month. The costs of producing channel catfish at different stocking densities were estimated. There were no significant differences (P &gt; 0.05) as a result of stocking density among treatment means of (1) gross or net yields, (2) mean weights at harvest, and (3) growth or survival of fingerlings (24–36%) and carryover fish (77–94%). Mean and maximum daily feeding rates ranged from 40 to 53 kg/ha/d and 123 to 188 kg/ha/d, respectively, and feed conversion ratios averaged 1.75. There were no differences in any feed‐related parameter as a result of density. Water quality variables showed few differences among densities at samplings and no differences when averaged across the production season. Yield of fingerlings increased as stocking density increased with significant differences between the two highest and the two lowest stocking densities. Breakeven prices were lower at the higher stocking densities as a result of the higher yield of understocked fish and similar mean individual fish weights produced at these higher stocking densities. Overall, varying stocking densities of fingerlings in multiple‐batch systems had little effect on production efficiency and water quality. Additional research on managing the population structure of carryover fish in commercial catfish ponds may be warranted.

Dietary yeast culture supplementation improves initial rearing of McConaughy strain rainbow trout

The addition of a yeast culture food supplement (DVAqua from Diamond V. Mills, Cedar Rapids, IA, USA) was evaluated during feeding of McConaughy strain rainbow trout Oncorhynchus mykiss. Two trials were run consecutively, starting with initial feeding, and were replicated over 2 years. The yeast culture was added to commercial trout starter at levels of 0.125 and 0.25 g kg−1. The number of rainbow trout that died during the first 4 weeks of rearing was significantly reduced in the tanks receiving either of the yeast culture-supplemented diets compared with the tanks receiving a control starter diet. Ending tank weights and gain were significantly and consistently greater in the tanks fed the 0.25 g kg−1 yeast culture diet. Individual fish weights were also significantly heavier in the fish-fed yeast culture containing diets in each year of the study at the end of 27 days. In the trials from 4 to 8 weeks postinitial feeding, mortality rates were significantly less only in the tanks receiving 0.25 g kg−1 yeast culture supplementation. Individual fish lengths and weights at the end of the second trial were significantly greater in the tanks receiving either level of yeast culture compared with the control diet only in 1 year. Total tank weights and feed conversion were not significantly different between any of the treatments. The inclusion of DVAqua yeast culture provided significant increases in trout survival and growth through the first 4 weeks of feeding, with continued benefits for the next 4 weeks.

Comparison of Two External Transmitter Types on Two Sizes of Bluegills and Yellow Perch

AbstractWe compared two external transmitter styles on two size‐classes of bluegills Lepomis macrochirus (laterally compressed) and yellow perch Perca flavescens (fusiform) to determine the effects of (1) even distribution of transmitter weight and (2) transmitter weights above and below the recommended 2% of fish body weight. Fish were collected, acclimated for 3 weeks before transmitter attachment, measured for length and weight, Floy tagged, and randomly assigned to treatments (single‐sided transmitter, saddle transmitter, and control). Weight and length of individual fish were recorded every 5 d through day 20 and every 10 d through day 40. On day 40, we classified tag migration, wound redness, and organosomatic indices. Saddle transmitters required the longest handling time of all treatments and did not outperform single‐sided transmitters. On day 40, only small yellow perch had gained more weight with saddle transmitters than with single‐sided transmitters. Transmitters that exceeded 2% of fish weight slowed bluegill growth, whereas transmitters that were less than 2% of fish weight negatively affected yellow perch growth. Our results suggest that even the distribution of external transmitter weight may be less important than the method of attachment and transmitter weight. Development of improved attachment methods for saddle transmitters may prove beneficial for use in long‐term studies of small, fusiform fish.

Water uptake in herring ( Clupea harengus) and sprat ( Sprattus sprattus) as a function of area, salinity and fat content

When pelagic fish is landed, the total weight includes both fish and water. In Sweden pelagic fisheries for herring ( Clupea harengus) and sprat ( Sprattus sprattus) are allowed to reduce the catch weight with 13% in the Baltic (2003) and 2% in Skagerrak/Kattegat and the North Sea. However, it has also been observed that the weight of individual fish increase due to fish absorb water during storage in the holding tanks onboard fishing vessels. Depending on the magnitude of the water uptake, the catch and landing statistics may be seriously biased. Furthermore, water uptake can also affect our perception of the variations in fish condition related to changes in the environment. Hence in this study, we explore the water uptake in different sized herring and sprat from three areas around the Swedish coast differing in salt concentration. We quantified the water uptake over time in herring and sprat stored fresh in tanks as a function of salinity and fat content. Salinity and fat content was used as covariates since osmotic potential and fat content has been suggested to affect water uptake in fishes. The results indicate that water uptake did not differ between species but did as a function of area and size (length). When exploring the two species separately, water uptake in herring was affected by area and salinity whereas water uptake in sprat by area and size (length). The water uptake difference between the two species was probably related to the stronger relationship between fat content and size (length) in sprat, compared to herring. Overall, the results suggest that the water reduction index used in Sweden as well as other countries should be changed.

Seasonal growth patterns of wild juvenile fish: partitioning variation among explanatory variables, based on individual growth trajectories of Atlantic salmon (Salmo salar) parr

We present an empirical, analytical model that estimates both temperature and seasonal response functions for the growth of wild juvenile fish without the need for costly tank experiments in less realistic conditions. 2. Analysis of monthly recapture data on the lengths and weights of individual wild juvenile fish demonstrates that simple temperature-driven models of growth can be less informative than more realistic, empirical, models. 3. A case study of wild Atlantic salmon parr (Salmo salar) showed that: most growth took place in a 10-week period in spring, at temperatures below those that previous published models report as necessary for rapid growth and at faster rates than the maximum that previous models predicted. 4. Temperature and fish size allometry explained 45% of growth variation, but the effects of temperature were significantly and markedly different at different seasons. 5. Seasonal effects explained an additional 18% of the variation and were strongly associated with the abundance of potential 'drift' food. 6. The seasonal patterns for growth in length and weight were different, implying differential allocation of resources to structural and reserve tissues. 7. The growth patterns of sexually maturing male parr and emigrants also differed in comparison to other parr. 8. Condition factor, length at first capture and seasonal interactions with both early maturity and smolting explained another 7% of the variation. 9. However, individual fish did not grow consistently better, or worse, than the 'average' fish. 10. This study emphasizes the necessity to test the adequacy of laboratory-based physiological models with suitably detailed field data and to focus model refinement by identifying processes which otherwise confound interpretation.