Haematology and leucocyte morphology of wild caught Thunnus maccoyii

The haematology of wild southern bluefin tuna Thunnus maccoyii was described using blood samples collected from fish immediately after they were caught. Cytology and cytochemistry revealed that the blood in peripheral circulation is comprised of erythrocytes, reticulocytes, ghost cells, lymphocytes, thrombocytes, eosinophilic granulocytes, neutrophilic granulocytes and monocytes. Reference ranges established were 41·09–55·50% for haematocrit, 0·62–3·00% for leucocrit, 13·25–17·92 g dl⁻¹ for haemoglobin and 2·1–2·9 million erythrocytes μl⁻¹ for erythrocyte count. Differential cell counts showed 94·58 ± 2·15% erythrocytes, 3·99 ± 1·44% leucocytes and 1·43 ± 1·03% thrombocytes (mean ±  s . d .). Normal ranges for differential leucocyte counts were 0·00–5·45% for neutrophils, 0·69–12·06% for eosinophils, 0·00–5·03% for monocytes, 46·97–74·32% for lymphocytes and 14·47–43·92% for thrombocytes. Erythrocyte indices, leucocyte types and cytochemistry were comparable to other species of scombrids. Packed cell volume was sensitive to the physiological state of the fish and to sample handling technique.     

Host specificity of two species of Gnathia (Isopoda) determined by DNA sequencing blood meals

Host specificity data for gnathiid isopods are scarce because the parasitic stages are difficult to identify and host-parasite contact is often brief. We examined two common nocturnal species, Gnathia falcipenis and Gnathia sp. C, collected in light traps from two locations at Lizard Island on the Great Barrier Reef, Australia. Engorged third stage gnathiids were photographed and permitted to moult into adults to allow identification. We compared approximately 580 bp sequences of 16S mtDNA from blood meals with host sequences available on GenBank using BLASTn. Where homology was <98%, familial identity was investigated with neighbour-joining trees. All blood meal sequences (n=60) and homologous fish sequences (n=87) from GenBank were used in a Bayesian analysis, which identified all but three sequences to family. The host frequency distributions used by each species were significantly different; only four host families were shared. No gnathiids fed on elasmobranchs, blennies or apogonids, and most fed on host families whose representatives are typically large. Gnathia sp. C showed a distinct predilection for nemipterids. Gnathia falcipenis often parasitised sand-dwelling families, and unlike sympatric diurnal gnathiid species, it also frequently parasitised pomacentrids. We conclude that G. falcipenis and Gnathia sp. C operate as generalist micropredators with preferences.     

The effect of temperature and fish size on growth and feed efficiency ratio of juvenile spotted wolffish Anarhichas minor

The growth properties of juvenile spotted wolffish Anarhichas minor reared at 4, 6, 8 and 12° C, and a group reared under 'temperature steps', (T‐step) i.e . with temperature reduced successively from 12 to 9 and 6° C were investigated. Growth rate and feed efficiency ration was significantly influenced by temperature and fish size. Overall growth rate was highest at 6° C (0·68% day⁻¹) and lowest at 12° C (0·48% day⁻¹), while the 4 and 8° C, and the T‐step groups had similar overall growth rates, i.e . 0·59, 0·62 and 0·51% day⁻¹ respectively. Optimal temperature for growth ( T _{opt G} ) and feed efficiency ratio (T_{opt FCE}) decreased as fish size increased, indicating an ontogenetic reduction in T _{opt G} and T _{opt FCE}. The results suggest a T _{opt G} of juvenile spotted wolffish in the size range 135–380 g, dropping from 7·9° C for 130–135 g to 6·6° C for 360–380 g juveniles. The T _{opt FCE} dropped from 7·4° C for 120–150 g to 6·5° C for 300–380 g juveniles. A wider parabolic regression curve between growth, feed efficiency ratio and temperature as fish size increased, may indicate increased temperature tolerance with size. Individual growth rates varied greatly at all time periods within the experimental temperatures, but at the same time significant size rank correlations were maintained and this may indicate stable size hierarchies in juvenile spotted wolffish.     

Hypertension in Pagothenia borchgrevinki caused by X-cell disease

Approximately 15% of a population of the cryopelagic nototheniid fish Pagothenia borchgrevinki , found constantly swimming immediately beneath the annual fast ice, in McMudro Sound, Ross Sea, Antarctica, was affected by X-cell gill disease. This disease affected blood flow through the gill lamellae, and this in turn affected oxygen uptake. Exercise caused increases in heart rate and ventral aortic blood pressure. Heart rate increased from 15·1 ± 1·55 to 23·1 ± 0·93 beats min⁻¹ in healthy fish, with a similar increase (to 24·6 ± 0·26 beats min⁻¹) in X-cell-affected animals. In healthy fish, pressures rose with exercise (from 2·72 ± 0·11 to 3·75 ± 0·19 kPa) and then rapidly returned to resting levels during recovery. In X-cell fish pressures rose during exercise, but then continued to rise, to reach a high of 4·18 ± 0·13 kPa, close to the predicted maximum pressure able to be generated by these hearts. Recovery was rapid in healthy fish, but was prolonged in diseased animals. As they are constantly swimming, there is the potential that X-cell-affected fish suffer from chronic hypertension.     

Hematozoa of teleosts from Lizard Island, Australia, with some comments on their possible mode of transmission and the description of a new hemogregarine species

Little is known of the blood parasites of coral reef fishes and nothing of how they are transmitted. We examined 497 fishes from 22 families, 47 genera, and 78 species captured at Lizard Island, Australia, between May 1997 and April 2003 for hematozoa and ectoparasites. We also investigated whether gnathiid isopods might serve as potential vectors of fish hemogregarines. Fifty-eight of 124 fishes caught in March 2002 had larval gnathiid isopods, up to 80 per host fish, and these were identified experimentally to be of 2 types, Gnathia sp. A and Gnathia sp. B. Caligid copepods were also recorded but no leeches. Hematozoa, found in 68 teleosts, were broadly hemogregarines of 4 types and an infection resembling Haemohormidium. Mixed infections (hemogregarine with Haemohormidium) were also observed, but no trypanosomes were detected in blood films. The hemogregarines were identified as Haemogregarina balistapi n. sp., Haemogregarina tetraodontis, possibly Haemogregarina bigemina, and an intraleukocytic hemogregarine of uncertain status. Laboratory-reared Gnathia sp. A larvae, fed experimentally on brushtail tangs, the latter heavily infected with the H. bigemina-like hemogregarine, contained hemogregarine gamonts and possibly young oocysts up to 3 days postfeeding, but no firm evidence that gnathiids transmit hemogregarines at Lizard Island was obtained.     

Is rigor mortis the cause of post‐mortem shrinkage in juvenile Pagrus auratus?

An experiment demonstrated that post‐mortem shrinkage in fork length of juvenile pink snapper Pagrus auratus (179–262 mm) was the same in fish in which rigor mortis was surgically inhibited, as in intact fish, and averaged 5·5 ± 0·2 mm (mean ±  s.e ., range 2–9 mm) after 168 h.     

Short‐term freshwater exposure benefits sea lice‐infected Atlantic salmon

Atlantic salmon Salmo salar were infected with sea lice Lepeophtheirus salmonis (0·08 ± 0·007 sea lice g⁻¹) over a period of 4 h. Both infected and non‐infected fish were swim tested in sea water (SW) and fresh water (FW). The ventral aorta of each fish was fitted with a Doppler cuff in order to measure cardiac output, stroke volume and heart rate during swim testing. Blood samples were taken at rest and after exercise. Critical swimming speed of infected fish in SW (2·14 ± 0·08 body lengths, bl s⁻¹) was significantly lower ( P  < 0·05) than infected fish switched to FW (2·81 ± 0·08 bl s⁻¹) and non‐infected fish in SW (2·42 ± 0·04 bl s⁻¹) and FW (2·61 ± 0·08 bl s⁻¹). Cardiac and blood results indicated infected fish exposed to FW did experience stress, but relief from osmotic and ionic distress probably reduces energy expenditure, allowing the increase in performance. As the performance of sea lice‐infected fish improved upon transfer to FW, it is likely that heavily infected salmonids do return to FW to restore compromised osmotic and ionic balance, and remove sea lice in the process.     

Physiological impact of sea lice on swimming performance of Atlantic salmon

Atlantic salmon Salmo salar were infected with two levels of sea lice Lepeophtheirus salmonis (0·13 ± 0·02 and 0·02 ± 0·00 sea lice g⁻¹). Once sea lice became adults, the ventral aorta of each fish was fitted with a Doppler cuff to measure cardiac output ( ̇ ), heart rate ( f _H) and stroke volume ( V _S) during swimming. Critical swimming speeds ( U _crit) of fish with higher sea lice numbers [2·1 ± 0·1 BL (body lengths) s⁻¹] were significantly lower ( P  < 0·05) than fish with lower numbers (2·4 ± 0·1 BL s⁻¹) and controls (sham infected, 2·6 ± 0·1 BL s⁻¹). After swimming, chloride levels in fish with higher sea lice numbers (184·4 ± 11·3 mmol l⁻¹) increased significantly (54%) from levels at rest and were significantly higher than fish with fewer lice (142·0 ± 3·7 mmol l⁻¹) or control fish (159·5 ± 3·5 mmol l⁻¹). The f _H of fish with more lice was 9% slower than the other two groups at U _crit. This decrease resulted in ̇ not increasing from resting levels. Sublethal infection by sea lice compromised the overall fitness of Atlantic salmon. The level of sea lice infection used in the present study was lower than has previously been reported to be detrimental to wild Atlantic salmon.     

Mitochondrial DNA sequence divergence between two cryptic species of Lethenteron, with reference to an improved identification technique

Genetic divergences and population structures were examined in the cryptic Lethenteron sp. N and sp. S, based on mitochondrial DNA (mtDNA) cytochrome oxidase subunit I (CO I) region sequences. An improved method of discrimination between L . sp. N and sp. S was found using PCR, with diagnostic primers for each species-specific sequence in the mtDNA CO I region. Identification of 50 individuals of each species by this analysis was consistent with that by allozyme analysis of nuclear DNA. L . sp. N and sp. S, identified on the basis of diagnostic alleles at five allozymic loci, were independently grouped in a neighbour-joining (NJ) tree, with a large sequence difference (mean ±  s . d . = 9·10 ± 0·36%) between them. Within each species, the values of sequence divergence among localities were significantly higher in L . sp. S (1·61 ± 0·44%) than in L . sp. N (1·10 ± 0·48%). On the tree and nested clade analyses, several phylogenetic groups comprising geographically close localities were detected in the former, although scarcely detected in the latter, probably resulting from dispersal pattern differences between them.     

Effects of confinement stress of variable duration on the growth and microincrement deposition in the otoliths of Oreochromis niloticus(Cichlidae)

The effects of chronic confinement stress (1, 5 and 10 days) and of periodic blood sampling on somatic growth and the structure and growth of otoliths was studied in Oreochromis niloticus . During the study, the plasma concentrations of cortisol were measured at various times during the application of stress: they were significantly higher in confined fish than in control fish (mean ±  s . d . 3·40 ± 0·47 v . 1·26 ± 0·62 ng ml⁻¹, P  < 0·05) up to 5 days after the start of a 10 day stress period. The somatic growth (standard length, L _S, and mass) was affected by the confinement and by the sampling (from 16·21 ± 1·07 to 14·64 ± 1·15 cm for L _S, and from 173·31 ± 33·14 to 110·50 ± 29·48 g for mass). But the confinement masked the effect of the sampling on somatic growth. Tetracycline was injected at the start of the experiment to mark the otoliths, and showed that the short and long duration confinements led to a clear check in the pattern of primary increments in the otoliths. The number of primary increments deposited during the resting periods that followed each period of confinement was always less than the number of days that these periods lasted. No relation was found between the duration of confinement and the structure of the resulting checks. These results suggest that there is a disruption in the laying down of primary increments during periods of confinement resulting in an underestimation of their number compared to the actual number of days of growth. These results call into question the use of otolith primary increments as a means of estimating the age of Nile tilapia that have experienced periods of stress.     

Habitat-related differences in the abundance of parasites from a coral reef fish: an indication of the movement patterns of Hemigymnus melapterus

The prevalence and number of an undescribed Benedenia sp. on Hemigymnus melapterus were significantly greater on fish from the reef flat than from the slope at Heron Island, Great Barrier Reef. In contrast, the abundance of gnathiid isopods on the fish did not differ significantly between habitats. As monogeneans do not leave their hosts, the differences in parasite abundance between the habitats suggest that H. melapterus does not move between the reef flat and reef slope, habitats separated by only a few hundreds of metres. Benedenia may thus be a useful biological tag for following the small-scale movement patterns of coral reef fish.     

Hatch date‐dependent differences in early growth and development recorded in the otolith microstructure of Trachurus japonicus

Otolith microstructure of 308 juvenile jack mackerel Trachurus japonicus sampled in the southern Sea of Japan between June and September 2002 were examined by light microscopy. Hatch‐date distribution indicated a protracted spawning season from 16 January to 30 May 2002. The first secondary primordium (SP) was formed at ages 19–54 days, with a mean ±  s . d . of 30·4 ± 6·1. The number of SP in an otolith ranged from two to 15, with a mean of 6·1 ± 1·7. Age of the first SP formation, number of SP and increment width varied with hatch date. In general, late‐hatched fish were younger when the first SP formed, and had a higher number of SP and wider increments, than early‐hatched fish. As increment width is a linear function of somatic growth and formation of the first SP is associated with metamorphosis in jack mackerel, these variations in otolith microstructure indicated that rates of growth (as inferred from wider increments) and development (as inferred by younger age of first SP formation) were higher in late‐hatched fish relative to early‐hatched fish. Higher temperature experienced by late‐hatched fish during early life stages may contribute critically to their higher growth and development rates compared to early‐hatched fish.     

A histological study on the development of the digestive system of Pseudosciaena crocea larvae and juveniles

The ontogenetic development of the gut and accessory organs in large yellow croaker Pseudosciaena crocea was investigated using light microscopy from hatching up to the juvenile stage (40 days post hatch, dph). At 3 dph (mean ±  s . d ., 4·1 ± 0·1 mm total length, L _T), coinciding with the buccopharynx opening, larvae started to feed exogenously, and the gut consisted of a well‐developed buccopharynx, a partially‐differentiated oesophagus and an intestine divided in three regions (anterior intestine, intermediate intestine and rectum). Yolk reserves were not completely depleted at the onset of exogenous feeding, and a period of mixed nutrition was observed up to 6 dph (4·3 ± 0·1 mm L _T), when yolk was definitively exhausted. Important morphological changes occurred at the end of the larval period, coinciding with metamorphosis. At 17 dph (6·8 ± 0·6 mm L _T), pyloric caeca differentiated at the junction of the pyloric stomach and the anterior intestine. Gastric glands were first observed at 21 dph (9·2 ± 1·2 mm L _T), coinciding with the morphological development of the stomach in three different regions (cardiac, fundic and pyloric) according to the histological characteristics of their mucosa. At this age, large longitudinal folds appeared in the median and posterior oesophageal mucosa. These morphological and histological features suggested the achievement of a digestive system characteristic of large yellow croaker juveniles and adults.     

Is white muscle anaerobic glycolysis capacity indicative of competitive ability in Arctic charr?

Food ingestion to assess dominance status, indicated that dominant Arctic charr Salvelinus alpinus displayed significantly higher white muscle pyruvate kinase (PK) and lactate dehydrogenase (LDH) activities compared to subordinate fish [PK = 429·57 ± 76·37 v . 331·00 ± 61·73 U g⁻¹ tissue ( P  = 0·022) and LDH = 929·33 ± 61·03 v . 775·25 ± 110·96 U g⁻¹ tissue; ( P  = 0·0007)].     

Predator‐induced hyperventilation in wild and hatchery Atlantic salmon fry

Following exposure to a predator stimulus (a brown trout Salmo trutta ), the opercular rate of Atlantic salmon Salmo salar fry increased by 35·3 ± 11·0%(mean ± 95% CI). The time taken for opercular rate to decline to baseline levels depended upon the occurrence of any associated locomotory activity. Opercular rates of fish that dashed when exposed remained elevated for 38·2 ± 20·6 min, whereas those of individuals that did not move ('freezers') recovered within 7·2 ± 2·9 min. The duration that opercular rate remained elevated was positively correlated with the magnitude of the elevation, which was higher in 'dashers' than freezers. The maximum opercular rate in 'freezers' was similar between wild fry and hatchery‐reared fry (from wild parents). There was a significant delay, however, in hatchery compared with wild fry in the time until peak ventilatory response and onset in the decline phase. This difference in opercular response suggests that hatchery fish were slower to realize fully the potential danger from the predator. Any delay in response could be directly attributed to the effect of hatchery‐rearing environment, rather than domestication or hatchery selection effects.     

Endurance swimming of diploid and triploid Atlantic salmon

When groups of diploid (mean ±  s . e . fork length, L _F) 33·0 ± 1·4 cm and triploid (35·3 ± 0·5 cm) Atlantic salmon Salmo salar were forced to swim at controlled speeds in a carefully monitored 10 m diameter 'annular' tank no significant difference was found between the maximum sustained swimming speeds ( U _ms, maintainable for 200 min) where the fish swam at the limit of their aerobic capability. Diploids achieved 2·99 body lengths per second (bl s⁻¹)(0·96 m s⁻¹) and triploids sustained 2·91 bl s⁻¹(1·02 m s⁻¹). The selection of fish for the trials was based on their ability to swim with a moving pattern projected from a gantry rotating at the radius of the tank and the selection procedure did not prove to be significant by ploidy. A significant difference was found between the anaerobic capabilities of the fish measured as endurance times at their prolonged swimming speeds. During the course of the experimentation the voluntary swimming speed selected by the fish increased and the schooling behaviour improved. The effect of the curvature of the tank on the fish speeds was calculated (removing the curved effect of the tank increased the speed in either ploidy by 5·5%). Implications of the endurance times and speeds are discussed with reference to the aquaculture of triploid Atlantic salmon.     

Effects of cadmium chloride on the development of rainbow trout Oncorhynchus mykiss early life stages

The sub-chronic (28–56 days) effects of exposure to low concentrations of cadmium (Cd; 0·05, 0·25, 0·50 and 2·50 μg l⁻¹) shortly following fertilization on embryos, larvae and juvenile rainbow trout Oncorhynchus mykiss were examined. Premature hatching occurred at lower concentrations (0·05 and 0·25 μg l⁻¹ Cd), however, delayed hatching was seen in the 2·50 μg l⁻¹ Cd group, with >90% of hatching occurring on the last day of the hatching period. Larval growth was negatively affected by Cd exposure in a concentration-dependent manner. Larvae exposed to 2·50 μg l⁻¹ Cd were 13·9 ± 0·8% shorter in total length ( L _T) and weighed 22·4 ± 3·5% (mean ± s . e .) less than controls at the end of the exposure period. Plasma sex steroid concentrations (oestradiol in juvenile females and 11-ketotestosterone in juvenile males) were elevated (four- to 10-fold over controls) in exposed fish in both males and females, following 28 days of exposure to 0·05, 0·25 and 0·50 μg l⁻¹ Cd, respectively. These results suggest that environmentally realistic concentrations (in the μg l⁻¹ range) of Cd can affect the development of O. mykiss impacting embryos, larvae and juvenile fish.     

Growth rate and nutritional status of wild and released reared juvenile turbot in southern Kattegat, Denmark

More than 800 000 reared juvenile turbot Psetta maxima were released in Danish waters during the period 1990 to 2002, as part of a stock enhancement programme. Performance of the released fish has been investigated using a combination of surveys and experimental studies. A total of 1278 wild fish and 439 alizarine marked released fish were caught in 16 surveys from 1993 to 1997. Recaptures were made over a period of c . 2 years after release. Results of growth analysis and measurements of nutritional status (condition factor, liver index, liver dry matter and otolith residual mass) showed only minor differences between wild and released fish. Growth rates of wild and of released fish were comparable to fish kept in the laboratory at the same temperature (specific length growth rate c . 0·5% day⁻¹ at 75–185 mm total length and 12·5° C). The results indicated sufficient food in the release area, but significant differences in growth and in nutritional status, between  years, were also observed. Nutritional status of both wild and released fish was comparable to fed fish in the experiments and significantly higher than in fish starved for  > 1 week. The effects of the transport and release procedures used were examined by simulated releases in laboratory tanks. The fish were observed to be feeding and growing normally during the first week after simulated release, suggesting this species to be relatively robust and unaffected by the handling and release procedures.     

Spawning characteristics of the anadromous brown trout in a small Swedish stream

The spawning pattern of the anadromous brown trout Salmo trutta was studied in Själsöån, a small stream in Gotland, Sweden, during eight winters between 1992–1993 and 1999–2000. The total length ( L _T) at spawning was normally distributed (185–890 mm) for females and multimodal for males (300, 400 and 550 mm most frequent length classes). Spawning males were significantly younger (2+ to 4+ years) than females (3+ to 5+ years). The sex‐ratio at the beginning and at the end of the spawning season favoured males. The mean ±  s . d . number of spawners was 70 ± 16 individuals per year. Migration into and out of the stream occurred between November and June. The highest number of spawning fish was found in the stream at the end of November or at the beginning of December. Migration mainly occurred during high water flow and at night. The majority of the females entered the stream and spawned the same (29·3% of all the females) or the next night (32·8% of all the females) while males may have stayed for 2 to 3 weeks (21·3% of all the males) in the stream before spawning. Males usually remained much longer in the stream (mean ±  s . d . 45 ± 56 days) than females (16 ± 30 days). Females lost more mass in the stream (mean ±  s . d . 17·3 ± 8·6%) than males (7·7 ± 9·6%). For both sexes, mass loss was positively correlated with the time spent in the stream. Only 7·3% of the males and 5·7% of the females occurred in the stream for >1 year. Spawning took place only during the night.