Abundance patterns of the toxic phytoplankton in coastal waters of the Balearic Archipelago (NW Mediterranean Sea): a multivariate approach

The prevalence of catch-and-release factors known to adversely affect the mortality or physical condition of Australian bass Macquaria novemaculeata were surveyed across a range of anglers throughout impoundments and rivers in New South Wales. Subsamples of tournament-caught fish were also monitored in tanks for 1 h after being weighed to quantify immediate mortalities and sublethal physical damage. Most fish were caught on actively fished artificial baits, and were mouth hooked (96%), which resulted in no immediate mortality amongst monitored fish and a low frequency of mortality-causing factors in surveys. However, fish that were weighed-in during tournaments often had fin damage (52% of fish) and/or barotrauma (25% of fish, in impoundments only), and were held in live wells with poor water-quality. The prevalence of these sublethal effects varied considerably according to the specific seasons, locations, rules and procedures of each tournament; but could not be reliably attributed to any of the recorded catch-and-release variables (except for dissolved oxygen, which was significantly influenced by live-well volume). While these results validate the current release of angled Australian bass as a means of conserving their stocks, the potential for adverse effects could nevertheless be minimised via simple changes to conventional handling.     

Mortality and physical damage of angled-and-released dusky flathead Platycephalus fuscus

We completed 2 experiments to quantify the mortality and physical damage (fin, blood and scale loss) of angled dusky flathead Platycephalus fuscus during a live weigh-in tournament (Expt 1) and after being immediately released by anglers (Expt 2). In each experiment, 84 and 79 angled P. fuscus were placed into up to 6 replicate tanks and, along with appropriate numbers of controls, monitored for mortalities over 5 d. Five and 7 of the angled fish died within 12 h, providing mortalities of 3.6 and 8.9% in Expts 1 and 2, respectively. One control fish died, which was attributed to an incorrectly inserted tag. None of the continuous or categorical variables collected during angling and subsequent release, or the experimental design, could explain the few observed mortalities to treatment fish. However, knotted large-mesh landing nets caused significantly greater fin damage than knotless fine-mesh designs in both experiments (p < 0.05). Furthermore, although water quality had no measured impact on the confined fish, the samples taken from anglers' live wells (mostly for Expt 1) were significantly poorer than the environment from which the fish were caught (p < 0.05). These latter differences may have had a cumulative negative impact on fish that manifested as the observed deaths. Simple changes to the post-capture handling of P. fuscus, involving appropriate landing nets and live wells, should reduce some angling impacts on this species.     

Resilience of inshore, juvenile snapper Pagrus auratus to angling and release

This study assessed the mortality of 157 snapper Pagrus auratus (9-29 cm, total length, L(T) ) after being conventionally angled and then released into cages (along with 48 controls) for 4 days off south-eastern Australia. Fatalities were restricted to 12 angled fish (7·6%) and mostly attributed to the ingestion of hooks and especially their subsequent removal, which caused substantial blood loss and immediate death. Hook ingestion was significantly biased towards smaller fish (<21 cm L(T)) and attributed to a lower chance of anglers initially detecting these individuals on the line (allowing them to consume more of the baits). While mortalities might be reduced in future via (1) choosing terminal rigs that promote mouth hooking and (2) cutting the line on any-hook ingested fish, the results nevertheless validate releasing unwanted angled inshore juvenile P. auratus as a means for managing their exploitation.     

Angling-Induced Barotrauma in Snapper Chrysophrys auratus: Are There Consequences for Reproduction?

In response to concerns regarding the potential for sub-lethal impacts of barotrauma on reproductively active Chrysophrys auratus during catch and release, 90 males and 90 females representing five reproductive stages (immature or resting-28%, developing-8%, developed-7%, ripe or spawning-23% and spent-34%) were angled from 8–70 m and macroscopically assessed (on-board and then in a laboratory). Irrespective of sex, all fish exhibited various clinical signs of barotrauma, including a prolapsed cloaca (60% of fish); gastric herniation (46%); ruptured swim bladder (73%); organ displacement (48%); and kidney (3%), liver (73%) and coloemic-cavity haemorrhaging (33%);with the frequency of nearly all positively associated with capture depth. Reproductive stage was also an important barotrauma predictor (reflecting related morphological changes) with a general trend towards spent fish least likely to incur the various clinical signs—especially for a prolapsed cloaca (also common among immature or resting fish and significantly affected by food in the digestive tract) and a ruptured swim bladder (common among ripe or spawning fish). The only macroscopically visible gonad damage was haemorrhaging, which was least common among immature or resting and spent fish and, irrespective of reproductive stage, temporally reduced in frequency, and more quickly among males than females. While further research is required to accurately describe the effects of angling at each stage of the reproductive cycle and the physiological consequences of barotrauma on the gonads of C. auratus, given the observed influences of reproductive stage and depth on barotrauma found in this study, any adverse effects might be partially managed by regulating either temporal or spatial fishing effort.     

Sublethal effects of angling and release on golden perch Macquaria ambigua: implications for reproduction and fish health

The present study tested the hypothesis of no delayed sublethal effects of mild angling and release on the feeding, growth, somatic condition and gonadal development of golden perch Macquaria ambigua during gametogenesis. Subsamples of adult M. ambigua (n = 17–21 of 207), originally captured from the wild and stocked into ten 0·1 ha earthen ponds, were angled and released during early and late gametogenesis. Wild samples that were concurrently collected throughout the experiment underwent rapid and synchronous gonadal development and many spawned. While no spawning occurred in the ponds, most M. ambigua underwent normal gonadal development to maturity, including the angled fish. Angled fish also fed, maintained condition and actually grew faster than non‐angled captive controls. Although females that were angled during late gametogenesis more readily ingested and retained baited hooks, neither their subsequent condition nor gonadal development was significantly affected. The predominance of null results was attributed to the combined effects of the flexible reproductive strategy of M. ambigua, the benignness of mouth hooking and immediate release, and possible methodological issues arising from differential hooking success of more aggressive and resilient individuals. The findings support earlier catch‐and‐release research, but contrast with reports of acute reproductive effects following capture and handling for aquaculture broodstock. This discrepancy highlights the need for research to specifically address welfare questions relevant to recreational fisheries across various species and angling scenarios.     

A hybrid zone and bidirectional introgression between two catadromous species: Australian bass Macquaria novemaculeata and estuary perch Macquaria colonorum

The presence and distribution of hybrid individuals and the existence of a hybrid zone between the catadromous Australian bass Macquaria novemaculeata and estuary perch Macquaria colonorum were investigated throughout the range of both species in Australia. Bayesian analyses and genotypic simulations identified 140 putative hybrids (11·5% of the total sample) with varying levels of introgression. Most hybrids were observed in an area extending from the Snowy River to the Albert River suggesting a hybrid zone in the eastern Bass Strait region. Sixteen hybrids, however, were found outside this zone, possibly reflecting the movement of hybrid offspring between estuaries or their inadvertent release during fish stocking programmes. Biparental backcrossing was found to occur suggesting that hybrids were fertile. These results have implications for the management of the extensive stocking programme in M. novemaculeata and for understanding the potential role of habitat degradation and reduced water flow in facilitating hybridization in species with migratory life histories.     

Generalist niche, specialist strategy: the diet of an Australian percichthyid

Dietary analysis revealed that an impoundment population of Australian bass Macquaria novemaculeata holds a generalist niche, but one arising from persistent individual specialization and interindividual variation. This 'individual specialist' strategy appeared adaptive, but the strength of individual specialization was largely independent of variation in diet composition, except during blooms of Daphnia sp. Diet composition and dietary overlap showed only moderate ontogenetic variation, and niche breadth showed no relationship with ontogeny. Macquaria novemaculeata showed an asymmetric predator and prey size distribution, consistent with many aquatic predators, with positive relationships between fish size and average, maximum and minimum prey size. There was no asymmetry in the relative size-based niche breadths of individuals, however, which indicates that the niche is a fixed 'window' of relative prey sizes. The difference in the dietary niche and prey-size relationships of M. novemaculeata at the population and individual levels highlights the necessity of assessing the niche at both these levels.     

Effects of angler-induced exercise and air exposure on the mortality of mouth-hooked yellowfin bream (Acanthopagrus australis)

Two aquaria experiments were done to quantify the short-term (up to 10 days) mortality of mouth-hooked yellowfin bream ( Acanthopagrus australis ) after short and long playing times (5 vs 30 s) followed by different extremes in air exposure (2.5 vs 5 min). During exp. 1, 44 fish were angled from randomly-selected 5000-L holding tanks, played for 5 s, subjected to either 2.5 or 5 min air exposure and then released into replicate sea cages located in a 1050 KL pool. Appropriate numbers of controls were transferred without air exposure from an unfished 5000-L tank into separate cages in the pool. The same methodology was applied during exp. 2, except that 31 yellowfin bream were angled and played for 30 s (rather than 5 s) before being subjected to the different durations of air exposures and released. Fish were monitored for 10 and 5 days in exps. 1 and 2, respectively. Concentrations of plasma cortisol and glucose were collected from samples of fish on the first and last days of the experiments. The only mortalities were two individuals (one from each treatment) during exp. 2, attributed to clotted blood (from hook wounds) across the gill lamellae. Surviving individuals at the end of the experiments had variable elevations in plasma cortisol but not glucose, indicating acute stress responses that were probably evoked during sampling. The conclusion is that yellowfin bream are resilient to being mouth-hooked, exercised and exposed to air before being released. However, to minimise impacts, individuals should be released as soon as possible, especially when blood is present at the hook wound.     

Release method and anatomical hook location: effects on short-term mortality of angler-caught Acanthopagrus australis and Argyrosomus japonicus

One field and 3 aquaria experiments were done to quantify the short-term mortality of yellowfin bream Acanthopagrus australis and mulloway Argyrosomus japonicus after being angled and subjected to 3 general handling treatments. Anglers were supplied with identical J-type hooks and asked to handle hooked fish by either (1) physically removing the hook or (2) cutting the line (5 cm from the mouth of the fish) and leaving the hook in. Some hooked A. japonicus were subjected to a third handling treatment where the line was cut underwater without exposing the fish to air. Technical and biological data were collected before all fish were released into sea cages and monitored for 5 d. Control fish were seined and similarly caged and monitored. Concentrations of plasma glucose and cortisol were collected from a sample of fish on the first and last day of the experiments. Significant predictors of mortality for both species involved the presence of blood at the mouth and an interaction between anatomical hook location and hook removal. A. australis and A. japonicus that had their ingested hooks removed experienced the greatest mortalities (87.5 and 72.7%, respectively). Typically, these fish suffered damage to their oesophagus, stomach wall and vital organs. Mortality rates of A. australis and A. japonicus were significantly decreased to 1.7 and 16%, respectively, when they were released with their lines cut, with some of these fish free of hooks after 5 d. In contrast, few mortalities occurred in either species when the hooks were removed or the lines cut on mouth-hooked fish or in A. japonicus when it was released with no air exposure. For A. australis, the field- and aquaria-based experiments provided comparable results in terms of identifying treatment-specific effects, but there were potential biases in rates of hook ingestion. Irrespective of the treatment of fish, all experiments caused physiological changes measured as elevations in either plasma cortisol or glucose. We concluded that anglers should cut the line from hook-ingested A. australis and A. japonicus, but remove the hook from mouth-hooked individuals to prevent subsequent ingestion. Further research is required to examine the longer-term consequences of these handling practices on fish health.     

Consequences of a catadromous life-strategy for levels of mitochondrial DNA differentiation among populations of the Australian bass, Macquaria novemaculeata

The influence of a catadromous life-strategy on levels of spatial genetic structuring in fish is poorly understood. In an effort to gain a better appreciation of how this specialized life-strategy determines population genetic structuring, we assessed variation in the mitochondrial DNA (mtDNA) control region in a catadromous perciform, the Australian bass Macquaria novemaculeata . Nineteen putative haplotypes were resolved using temperature gradient gel electrophoresis from 10 geographically distinct populations. Significant heterogeneity was revealed in haplotype frequencies and their spatial distributions among many locales. Gene partitioning statistics ( AMOVA ) for both raw haplotype frequency data and frequency data with sequence divergences were concordant, indicating that M. novemaculeata populations were moderately genetically structured (Φ_ST = 0.05, 0.06; P < 0.001, respectively). Isolation by distance seems to be a strong structuring force in M. novemaculeata , culminating in no detectable phylogeographic structuring among haplotypes. Low sequence divergences were observed among many haplotypes and it is suggested that these are the result of pruning of maternal lineages by cyclical variations in female reproductive success. This study highlights the importance of life-history patterns and, in particular, spawning locality, in determining spatial structuring of mtDNA variation in catadromous species.     

Ingestion and ejection of hooks: effects on long-term health and mortality of angler-caught yellowfin bream Acanthopagrus australis

Ninety juvenile yellowfin bream Acanthopagrus australis were angled from holding tanks, allowed to ingest nickel-plated, carbon-steel J-hooks and released (with their lines cut) into individual experimental tanks during 2 experiments in order to assess their (1) long-term (up to 105 d) health, mortality and rate of hook ejection and (2) short- and medium-term (< 42 d) temporal changes in health during hook ingestion. Equal numbers of control fish were scooped from holding tanks and similarly monitored in experimental tanks. Of 20 hook-ingested fish released during Expt 1, 3 died within 8 d, providing a non-significant mortality of 15%. Between Day 6 and Day 56 post-release, 13 of the surviving individuals ejected their hooks, which were typically oxidized to about 94% of their original weight and often broken into 2 pieces. At Day 105, there were no significant differences between the 20 control and 17 hook-ingested/-ejected fish in terms of their ability to digest and assimilate food (measured as changes in apparent digestibility coefficients), stress (measured as concentrations of plasma cortisol and glucose) or of morphological parameters that included weight (Wt) and maximum height (MH), maximum width (MW) and maximum girth (MG). During Expt 2, 3 individuals that still contained ingested hooks and 3 controls were sampled on each of 9 occasions between Day 3 and Day 42 post-release. All fish were sampled for blood cortisol and glucose and were then euthanized before being weighed and measured for total length (TL), MH, MW and MG. Hook-ingested individuals were also X-rayed to determine the position and orientation of hooks. There were no significant differences in plasma glucose between hook-ingested and control fish. Irrespective of the treatment of fish, concentrations of cortisol were elevated on some sampling occasions, indicating variable, acute stress. The MH and MG of fish were not significantly different between groups. Significant differences were detected for MG and Wt, with hook-ingested fish having weights similar to those of the control fish but a relatively greater MW (owing to stomach distension from ingested hooks) until 2 wk post-release, after which both morphological parameters generally declined. There was no significant temporal progression of hooks in the stomach of treatment fish; however, some hooks reorientated to positions that may have precluded passage along the digestive tract. We conclude that, for the J-hooks examined, cutting the line is an appropriate strategy that results in the greater majority of released hook-ingested yellowfin bream surviving with minimal negative long-term effects.     

Effects of angling on post‐release mortality,gonadal development and somatic condition of Australian bass Macquaria novemaculeata

This study assessed the protracted effects of two angling treatments (mild and harsh) on the post‐release mortality, gonadal development and somatic condition of Australian bass Macquaria novemaculeata in a pond‐based experiment. Angled fish were destructively sampled (along with controls) three times: immediately before, and 1 or 4 weeks after release into cages. Compared with the other groups, harshly angled fish had similar, low immediate and short‐term mortalities (≤5%), but significant delayed mortality (25%) and fed only minimally. None of these fish ejected ingested hooks. Forty‐six per cent of captive fish (across all groups) and 20% of wild fish had non‐ripening gonads (stage I) prior to, or during, the experiment. In females with ripening ovaries (stages II or III), neither angling treatment significantly reduced standardized gonadal mass. The mean per cent of atretic oocytes increased among females in all groups, but was significantly greater in those that were harshly angled. The results demonstrated that the gonadal development of M. novemaculeata could be suppressed or impaired (by angling, handling and confinement), and that further research is warranted. In particular, the timing and severity of angling in relation to the stage of gonadal development could have important implications for the introduction of temporal restrictions to angling.     

Effects of salinity and anatomical hook location on the mortality and physiological response of angled-and-released sand whiting Sillago ciliata

Three experiments were done with sand whiting Sillago ciliata: the first two assessed the short‐term mortality and physiological response of individuals after being mouth hooked and then subjected to rapid changes in salinity, while the third experiment investigated their longer‐term fate after ingesting hooks (independent of salinity changes). In experiment one, 48 tanks containing a single S. ciliata were randomly assigned as either one of three treatments or a control. The fish in treatments one and two were exposed to salinity changes during their angling and subsequent release while those in treatment three were only subjected to angling and air exposure. Control fish remained untouched. Fish were then monitored for up to 6 days for mortalities before blood samples were taken to determine concentrations of plasma cortisol and glucose. Blood samples were also taken from five wild‐caught fish to provide baseline estimates of the above variables. None of the treatment or control fish died over the 6 days, and there were no significant differences in blood cortisol or glucose between treatment, control and wild fish. In experiment two, 102 S. ciliata and 52 experimental tanks were used. The treatments were repeated as above, however, six individuals from each treatment and control group were removed and sampled for blood (and then glucose and cortisol) at 0, 24, 48 and 72 h post release. Some changes in behaviour due to the salinity changes occurred as well as a significant main effect of time for cortisol, with all fish having significantly elevated acute stress at the first sample time. In experiment three, 52 S. ciliata were placed into individual tanks. Twenty‐six of these fish were allowed to ingest baited J‐hooks, played for 60 s, removed from their tanks, and then released after their lines were cut (50 mm from their mouths). Control fish were not touched. All fish were then monitored over 21 days. Six of the treatment fish died (between 3 h and 14 days), while the remaining hooked fish resumed feeding within 5 days and 25% ejected their hooks (between 1 and 19 days). It was concluded that (1) salinity and mouth hooking had few independent or interactive effects on the mortality or physiological response of angled‐and‐released S. ciliata and (2) while hook ingestion caused some mortalities, the protracted physiological effects were limited, with all surviving fish resuming feeding and some eventually ejecting their hooks.     

Mortality of key fish species released by recreational anglers in an Australian estuary

A field experiment was done to quantify the mortality of fish released during a recreational angling tournament in Botany Bay, Australia. Participating boat-based anglers were divided into two groups, each representing different typical catch-and-release events. The first group (termed the ‘live weigh-in group’) retained the largest two individuals of 4 species (dusky flathead, Platycephalus fuscus, yellowfin bream, Acanthopagrus australis, sand whiting, Sillago ciliata, and trevally, Pseudocaranx dentex) in onboard holding tanks and then presented these to researchers at designated weigh-in times and stations. Gear, operational and handling data were collected before 125 fish were tagged using plastic t-bar tags, returned to the anglers and then released into two sea cages. The second group (termed the ‘immediate-release group’) immediately released 224 fish into two sea cages, after they were tagged and relevant data recorded by onboard observers. This group represented those fish routinely discarded (i) as part of catch-and-immediate-release tournaments and/or (ii) due to minimum legal sizes and/or personal quotas. Appropriate species and numbers of ‘control’ fish were seined and placed into two sea cages. All fish were monitored for mortalities over 10 days. Dusky flathead, yellowfin bream, trevally and snapper, Pagrus auratus accounted for more than 85% of the total catch. Their adjusted mortalities ranged between 0% and 36.6%. Irrespective of the treatment, most yellowfin bream and snapper deaths occurred within 3 h of being hooked and released into the cages, while trevally and dusky flathead showed a delayed mortality over 4 days. Owing to confounding effects due to their confinement, dusky flathead were excluded from further analyses. Anatomical hook location and the time between capture and release were significant predictors of mortality for yellowfin bream and trevally, respectively (p < 0.01), but none of the various gear, operational or handling factors examined were significant for snapper (p > 0.05). The results are discussed in terms of species-specific variabilities in mortalities, their causal effects and better management of catch-and-release events.     

Recovery of barotrauma injuries in Chinook salmon, Oncorhynchus tshawytscha from exposure to pile driving sound

Juvenile Chinook salmon, Oncorhynchus tshawytscha, were exposed to simulated high intensity pile driving signals to evaluate their ability to recover from barotrauma injuries. Fish were exposed to one of two cumulative sound exposure levels for 960 pile strikes (217 or 210 dB re 1 μPa(2)·s SEL(cum); single strike sound exposure levels of 187 or 180 dB re 1 μPa(2)?s SEL(ss) respectively). This was followed by an immediate assessment of injuries, or assessment 2, 5, or 10 days post-exposure. There were no observed mortalities from the pile driving sound exposure. Fish exposed to 217 dB re 1 μPa(2)·s SEL(cum) displayed evidence of healing from injuries as post-exposure time increased. Fish exposed to 210 dB re 1 μPa(2)·s SEL(cum) sustained minimal injuries that were not significantly different from control fish at days 0, 2, and 10. The exposure to 210 dB re 1 μPa(2)·s SEL(cum) replicated the findings in a previous study that defined this level as the threshold for onset of injury. Furthermore, these data support the hypothesis that one or two Mild injuries resulting from pile driving exposure are unlikely to affect the survival of the exposed animals, at least in a laboratory environment.     

Controls on the Entrainment of Juvenile Chinook Salmon (Oncorhynchus tshawytscha) into Large Water Diversions and Estimates of Population-Level Loss

Diversion of freshwater can cause significant changes in hydrologic dynamics and this can have negative consequences for fish populations. Additionally, fishes can be directly entrained into diversion infrastructure (e.g. canals, reservoirs, pumps) where they may become lost to the population. However, the effect of diversion losses on fish population dynamics remains unclear. We used 15 years of release and recovery data from coded-wire-tagged juvenile Chinook Salmon (Oncorhynchus tshawytscha) to model the physical, hydrological and biological predictors of salvage at two large water diversions in the San Francisco Estuary. Additionally, entrainment rates were combined with estimates of mortality during migration to quantify the proportion of total mortality that could be attributed to diversions. Statistical modeling revealed a strong positive relationship between diversion rate and fish entrainment at both diversions and all release locations. Other significant relationships were specific to the rivers where the fish were released, and the specific diversion facility. Although significant relationships were identified in statistical models, entrainment loss and the mean contribution of entrainment to total migration mortality were low. The greatest entrainment mortality occurred for fish released along routes that passed closest to the diversions and certain runs of Chinook Salmon released in the Sacramento River suffered greater mortality but only at the highest diversion rates observed during the study. These results suggest losses at diversions should be put into a population context in order to best inform effective management of Chinook Salmon populations.     

Effects of fish age and parasite dose on the development of whirling disease in rainbow trout

We determined the ages at which juvenile rainbow trout Oncorhynchus mykiss became resistant to the effects of whirling disease following exposure to a range of parasite doses. Heretofore, the development and severity of whirling disease in salmonids was known to be generally dependent on the age or size of fish when first exposed to the triactinomyxon spores of Myxobolus cerebralis; larger, older individuals tended to be less diseased. However, no systematic determination had been made of the exact age at which fish become resistant to the development of the disease. We exposed rainbow trout at 9 ages (1 to 17 wk post-hatch) to 4 parasite dose levels (0, 100, 1000 and 10,000 triactinomyxons per fish). Disease severity was measured using mortality, clinical signs, microscopic pathology, and myxospore counts. Disease and mortality were substantially reduced when exposure to the parasite occurred for the first time at 9 wk post-hatch (756 degree-days at 12 degrees C) or older. High doses elicited more disease among the younger age groups, but the effect was dampened in groups exposed at about 9 to 11 wk post-hatch and absent thereafter. Rainbow trout reared in M. cerebralis-free waters for 9 wk post-hatch or longer, whether in the wild or in a hatchery situation, should experience greater survival and less disease than fish first exposed to the parasite at younger ages.     

Effects of density on perching behavior of broiler chickens

Exercise through perching has been suggested as a way to reduce the impact of leg problems in broiler chickens. It is possible that higher stocking densities may motivate birds to perch more, and perhaps reduce some of the detrimental effects to broiler health seen with high stocking densities. The goal of this research was to investigate the effects of density (10, 15, and 20birds/m(2)) and perch design on the frequency of perch utilization. Mixed sex broilers were assigned to 36 pens in a four perch treatmentxthree density factorial with three replications for each treatment/density combination in a randomized complete block design. Pens were assigned to one of four perch treatments: control (no perches), horizontal (three horizontal perches), angled (three 10 degrees angled perches), or mixed angle (one horizontal, one 10 degrees angled, and one 20 degrees angled perch), at each of the three densities. The results show that although the frequency of perch use was low, (2.6%+/-0.15), significant preferences were clear. Perching frequency was significantly higher when birds were raised at stocking densities of 15 (2.88+/-0.303) or 20birds/m(2) (2.76+/-0.211), as opposed to 10birds/m(2) (2.10+/-0.209) as indicated by a marginally significant main effect (p<0.0597), and a significant linear effect (P<0.0427). The birds used the horizontal perches the most, followed by 10 degrees angled and mixed angle perch treatments with decreasing frequency. An effect of age was found, in that perching increased during the first 4 weeks, and dropped off significantly at the end of the rearing period for all perch treatments. In general, perches closest to the hallway or outside of the house were used more than perches in the middle of the pen. Preferential use of the highest section of the 10 degrees angled perches was also found. The applied perch treatments or densities did not significantly affect final body weight or feed conversion. Percent mortality due to heat stress showed a significant increase with density, while mortality not caused by heat stress did not change with perch treatment or density.     

The effect of hatchery release strategy on marine migratory behaviour and apparent survival of Seymour River steelhead smolts (Oncorhynchus mykiss)

Early marine migratory behaviour and apparent survival of hatchery-reared Seymour River steelhead (Oncorhynchus mykiss) smolts was examined over a four year period (2006-2009) to assess the impact of various management strategies on improving early marine survival. Acoustically tagged smolts were released to measure their survival using estuary and coastal marine receivers forming components of the Pacific Ocean Shelf Tracking (POST) array. Early marine survival was statistically indistinguishable between releases of summer run and winter run steelhead races, night and day releases, and groups released 10 days apart. In 2009, the survival of summer run steelhead released into the river was again trialed against groups released directly into the ocean at a distance from the river mouth. Apparent survival was improved significantly for the ocean released groups. The health and physiological status of the various release groups were monitored in years 2007-2009, and results indicate that the fish were in good health, with no clinical signs of disease at the time of release. The possibility of a disease event contributing to early marine mortality was further examined in 2009 by vaccinating half of the released fish against common fish diseases (vibriosis, furunculosis). The results suggest that marine survival may be enhanced using this approach, although not to the extent observed when the smolts were transported away from the river mouth before release. In summary, direct experimental testing of different release strategies using the POST array to measure ocean survival accelerated the scientific process by allowing rapid collection of data which enabled the rejection of several existing theories and allowed tentative identification of several new alternative approaches that might improve early marine survival of Seymour River steelhead.     

Density-dependent predation and implications for stock enhancement with Japanese flounder

In laboratory predation trials kinsen-gani crab Matuta lunaris rarely preyed on hatchery-reared (HR) Japanese flounder Paralichthys olivaceus , even at extremely elevated prey densities ( c . 90 fish m⁻²), but fed readily on sacrificed Japanese founder. This suggests that under similar conditions in the wild predator-prey interactions between kinsen-gani and juvenile Japanese flounder are likely to be weak. Larger Japanese flounder readily cannibalized smaller conspecifics, suggesting that within-release group cannibalism may contribute considerably to post-release mortality of HR Japanese flounder when sufficient size variation exists within groups of released fish. Kinsen-gani presence had no effect on within-release group cannibalism rates of large Japanese flounder on small Japanese flounder. Within-release group cannibalism rates were significantly affected by densities of smaller (prey) fish, implying that the number of fish released (and thus post-release densities) in stock enhancement programmes may significantly affect post-release mortality rates. The probability of within-release group cannibalism increased rapidly as the cannibal-to-prey size ratio exceeded 1.6, suggesting that minimizing size variation of released HR individuals will reduce post-release cannibalism rates. These results have implications for Japanese flounder stock enhancement release strategies, and may be applicable to other finfish stock enhancement programmes.