Seasonal variations in the prevalence and infestation intensity of Gyrodactylus salaris Malmberg, 1957 (Monogenea: Gyrodactylidae) on Atlantic salmon parr, Salmo salar L., in the River Batnfjordselva, Norway

The prevalence of Gyrodactylus suluris Malmberg. 1957 on both yearlings and older parr of Atlantic salmon ( Sulmo sulur L.) in the River Batnfjordselva was 100% through most of the year. With one exception, uninfested fish were only found in the winter and spring after the water temperature had fallen to almost 0° C for 2–3 months. In general, the abundance (i.e. the mean number of parasites per investigated fish) of G. salaris increased during the warm period of the year (summer and early autumn). Abundanceas high as 1153 and 4418 in early autumn was found on yearlings and older parr, respectively. The abundance decreased during the cold period of the year (winter and early spring), in some cases to as low as two and four G. salaris on yearlings and older parr, respectively. About 86% of the G. salaris specimens were found on the fins of the salmon parr: mainly on the dorsal fin (34.4%) and the pectoral fins (27.0%). The remainder of the parasites were distributed on the body (7.8%), the head (3.5%), and the gills (2.6%).     

Mitochondrial DNA variation of Gyrodactylus spp (Monogenea, Gyrodactylidae) populations infecting Atlantic salmon, grayling, and rainbow trout in Norway and Sweden

Approximately 800 bp of the mitochondrial cytochrome oxidase I (COI) gene were sequenced from 76 Gyrodactylus specimens of 32 salmonid host populations, i.e. from Salmo salar, Thymallus thymallus, and Oncorhynchus mykiss in Norway, Sweden and Latvia. The COI sequences indicated a substantial intraspecific differentiation of Gyrodactylus salaris and Gyrodactylus thymalli. In total, 12 haplotypes were identified which group into five well supported clades, three clades with parasites from Atlantic salmon and two clades with parasites from grayling. The basal nodes linking the five clades together are only weakly supported. Thus, there is no support for the monophyly of all G. salaris haplotypes and the monophyly of all G. thymalli haplotypes. The lack of monophyly of the mitochondrial haplotypes of G. salaris and G. thymalli may indicate that G. salaris and G. thymalli represent (i). two polytypic species or (ii). one polytypic species, or (iii). refer to a complex of more than two sibling species. The mtDNA data indicate multiple introductions of G. salaris and G. thymalli into Norway. A minimum of three independent introductions of G. salaris and two independent introductions of G. thymalli are supported. This is congruent with earlier hypotheses on the introduction of G. salaris and G. thymalli into Norway.     

The distribution of Cryptocotyle spp. metacercariae in the skin of caged Atlantic cod (Gadus morhua)

In caged Atlantic cod (>40cm), Cryptocotyle spp. metacereariae were concentrated on the dorsal surface, but relatively scarce on the fins. Behaviour and mechanical forces probably affect this distribution.     

Allometric growth in juvenile cod (Gadus morhua) and possible effects on cannibalism

Cod juveniles of about 20 mm had the lowest potential predator: prey length ratio based on mouth gape of the predator and the body height of the prey. This implies that the cannibalistic potential is maximized in this size range.     

Effects of temperature, hypoxia and activity on the metabolism of juvenile Atlantic cod

Standard metabolic rate (SMR), active metabolic rate (AMR) and critical oxygen saturation ( S_crit ) were measured in Atlantic cod Gadus morhua at 5, 10 and 15° C. The SMR was 35.5, 57.0 and 78.2 mg O₂ kg⁻¹ h⁻¹ and S_crit was 16.5, 23.2 and 30.3%, at 5, 10 and 15° C, respectively. Previously reported SMR for Atlantic cod from arctic waters at 4° C was twice that measured at 5° C in the present study. A possible intraspecific latitudinal difference in the SMR is discussed. The AMR was 146.6, 197.9 and 200.4 mg O₂ kg⁻¹ h⁻¹ and the critical swimming speed ( U_crit ) was 1 6, 1.7 and 1.9 at 5, 10 and 15° C, respectively. The maximum oxygen consumption was found to be associated with exercise, rather than recovery from exercise as previously reported in another Study of Cod metabolism.     

Evidence that size-selective mortality affects growth of Atlantic cod (Gadus morhua L.) in the southern Gulf of St Lawrence

The effects of size-selective fishing mortality on the growth of Atlantic cod ( Gadus morhuu L.) in the southern Gulf of St Lawrence were investigated and compared between: (1) a period when fishing mortality was relatively high, growth was relatively rapid, and abundance low (1967–1972 year classes): and (2) a period when fishing mortality was lower, growth was slow, and density high (1977–1982 year classes). Cod first entered the fishery at age 3 during both periods. The 1967–1972 year classes (fast growing) were fully recruited to the fishery by age 5 or 6 and the fishery removed over twice as many fish from the lower than upper quartiles of length-at-age distributions for cod 4 to 10 years old (disproportionately high exploitation of slow-growing fish). In contrast. the 1977–1982 year classes (slow growing) did not fully recruit to the fishery until age 9 or 10 and the fishery removed four times as many fish from the upper than lower quartiles of the length-at-age distributions for 4- to 10-year-old cod (disproportionately high exploitation of fast-growing fish). The reduced mean lengths-at-age of the 1977–1982 year classes compared with the 1967–I972 year-classes is consistent with the different patterns of exploitation of fast- and slow-growing fish for the two periods.     

A reappraisal of activity metabolism in Atlantic cod (Gadus morhua)

Atlantic cod ( Gadus morhua ) were forced to swim in a swim tunnel respirometer until fatigued; oxygen consumption rate (O₂) was measured during swimming at incremental speeds until the fish was exhausted and during recovery from exhaustion. Maximal oxygen consumption (O_2max) occurred during maximal activity as has been found for other fish species, but at odds with the current paradigm for Atlantic cod. Earlier experiments had drawn the conclusion that O_2max in Atlantic cod occurs during recovery from exhaustive exercise. We found no support for this paradigm in our experiments and we propose that the respiratory physiology of Atlantic cod is not unlike that of other fishes.     

Genetic variation and population structure of cod, Gadus morhua L., in some fjords in northern Norway

Biochemical genetic polymorphism in cod from several fjords in Troms, northern Norway was analysed. Gene frequencies at several polymorphic loci ( Hb, Pgi, Ldh, Pgm, Gpd and Idh ) are given. Significant variation was found both within and between fjords, and even between samples from the same locality sampled in different years, indicating a mosaic structure of the cod population in the area studied. The results of the haemoglobin variation are compared to similar results obtained more than 20 years ago in the same and adjacent areas. The biochemical genetic variation is discussed in relation to stability of gene frequencies, isolation mechanisms and migration patterns.     

Winter feeding ecology of cod (Gadus morhua) in a fjord of southern Norway

We investigated the winter feeding ecology of cod ( Gadus morhua ) in the Risørfjord and Flødevigen areas on the Skagerrak coast, southern Norway. Diets from the ice-covered Risørfjord were compared with diets from the more coastal Flødevigen area. In the Risørfjord area the diet featured numerically both decapods (56.0%) and fish (27.8%), but fish dominated by mass (75.1%). The most numerous dietary items from the Flødevigen area were decapods (48.1%) and isopods (30.6%), although fish still made up the bulk of the diet's mass (52.6%). Diets at Flødevigen shifted from winter to spring, as polychaetes became important numerically (67.8%) and also contributed substantially by weight (53.6%). Since cod feed opportunistically, differences between areas and seasons probably reflected differences in prey diversity and abundance. Prey size variability increased with increasing predator length, but maximum prey size relative to predator length was constant at about 9%. Prey fish increased in both length and numbers with the length of cod, particularly in the Risørfjord area, where fish were more important in the diet than at Flødevigen. However, the majority of fish consumed in both areas were small gobiids. Based on growth patterns observed in otoliths, with opaque zones (indicating faster growth) formed during winter, the quality and quantity of the winter diet may determine annual growth rates of cod on the Skagerrak coast. Fish and decapods constituted important, high energy food sources, and there was little indication that cod were food limited during winter in this area.     

The feeding, growth and behaviour of juvenile cod, Gadus morhua L., in cold environments

Most studies on feeding-related activities of fish in cold water (0° C) have not used eurythermal species or examined a broad suite of activities. In this study we report on the feeding, growth, behaviour and conversion-efficiency of 0 + Atlantic cod in response to water temperature and food availability. In one experiment, activity, opercular beat and growth decreased with decreasing temperature over the range 8·3–0·6° C. However, the conversion efficiency increased with decreasing temperature, a result partly explained by the relative change in activity and opercular rate. In another experiment, temperature had a linear effect on growth in cod fed different food rations. Both food availability and temperature were found to influence growth in group 0 + cod. The specific growth rate of cod in this experiment exceeded 2% day^-1. Growth data collected from cod sampled in the field during the cold-water period showed a progressive increase of mean length over the season. Overall, these results demonstrate that 0 + cod have adapted to life in cold-water environments, in contrast to adult cod which mostly inhabit warmer environments.     

The demography of a haemoglobin polymorphism in the Atlantic cod, Gadus morhua L.

Genetic stability was investigated at a polymorphic haemoglobin gene-locus in 13 124 Atlantic cod, Gadus morhua L. A total of 30 year-classes, sampled over 18 years, were variously sampled in 33 named localities. Regional populations showed distinctive allele frequencies, and each major cod fishery showed some degree of genetic imbalance, which was expressed as excessive numbers of homozygotes. This imbalance occurred seasonally in a proportion of the population samples in each of the major cod fisheries. The findings are attributable to migrations of distinctive genetic populations.     

Buoyancy variations in eggs of Atlantic cod (Gadus morhua L.) in relation to chorion thickness and egg size: theory and observations

Egg specific gravity is a function of egg volume, chorion volume, perivitelline space (PVS) and the specific gravity of chorion and ovoplasm. Experimental studies on cod ( Gadus morhua L.), a marine multiple batch spawner, showed that approx. forty per cent of the chorion volume is incorporated between batches and that the chorionic material has a mean specific gravity of 1·20 (range: 1·14–1·35). The chorion volume and specific gravity are low at the end of the spawning period in extremely fecund fish. Specific gravity of ovoplasm was observed to be 1·017 in marine eggs and calculated to be about 1·008 in brackish water eggs. Inherent variation in PVS is found to have little influence on egg specific gravity. With the other variables constant, egg specific gravity is inversely related to egg volume supported by studies on single batches of eggs. Measurements on every batch of eggs reveal that the production of large eggs (>1·3 mm) gives low density eggs initially and high density eggs towards the end of spawning, while the production of small eggs (<1·3mm) gives moderate specific gravity. These observations are discussed in relation to vertical distribution of the eggs and reproductive success of recruit and repeat spawners. There are indications that repeat spawners produce larger eggs.     

Potential fitness benefits from mate selection in the Atlantic cod (Gadus morhua)

Little evidence of benefits from female mate choice has been found when males provide no parental care or resources. Yet, good genes models of sexual selection suggest that elaborated male sexual characters are reliable signals of mate quality and that the offspring of males with elaborate sexual ornaments will perform better than those of males with less elaborate ornaments. We used cod (Gadus morhua L.), an externally fertilizing species where males provide nothing but sperm, to examine the potential of optimal mate selection with respect to offspring survival. By applying in vitro fertilizations, we crossed eight females with nine males in all possible combinations and reared each of the 72 sib groups. We found that offspring survival was dependent on which female was mated with which male and that optimal mate selection has the potential to increase mean offspring survival from 31.9 to 55.6% (a 74% increase). However, the size of the male sexual ornaments and sperm quality (i.e. sperm velocity and sperm density) could not predict offspring survival. Thus, even if there may be large fitness benefits of mate selection, we might not yet have identified the male characteristics generating high offspring survival.     

Time of start of spawning in Atlantic cod (Gadus morhua) females in relation to vitellogenic oocyte diameter, temperature, fish length and condition

Spawning time of female Atlantic cod ( Gadus morhua L.) can be predicted to within 3 days using vitellogenic oocyte diameters measured over the last month before spawning. Simulations show that a l°C drop in temperature during vitellogenesis delays spawning by about 8–10 days. Similar results were obtained with fish on low ration for 4–8 months before spawning. Moderately fed females held over two to four consecutive spawning seasons demonstrated a significant delay (⋍3–40 days) in the date of first spawning as the fish grew older and more fecund. Forecasting studies on spawning migrating Arcto-Norwegian cod gave no evidence of a significant effect of fish length on the time of spawning. Warmer water temperature during vitellogenesis for the larger fish appears to compensate for the negative effect of fecundity On spawning time.     

A SNP/microsatellite genetic linkage map of the Atlantic cod (Gadus morhua)

A first genetic linkage map of the Atlantic cod ( Gadus morhua ) was produced, based on segregation data from 12 full-sib families of Norwegian origin. The map contained 174 single nucleotide polymorphism markers and 33 microsatellites, distributed on 25 linkage groups and had a length of 1225 cM. A significant difference in recombination rates between sexes was found, the average ratio of female:male recombination rates being 1.78 ± 1.62 (SD).     

Natural selection and the genetic differentiation of coastal and Arctic populations of the Atlantic cod in northern Norway: a test involving nucleotide sequence variation at the pantophysin (PanI) locus

To examine the role of contemporary selection in maintaining significant allele frequency differences at the pantophysin (PanI) locus among populations of the Atlantic cod, Gadus morhua, in northern Norway, we sequenced 127 PanIA alleles sampled from six coastal and two Barents Sea populations. The distributions of variable sites segregating within the PanIA allelic class were then compared among the populations. Significant differences were detected in the overall frequencies of PanIA alleles among populations within coastal and Arctic regions that was similar in magnitude to heterogeneity in the distributions of polymorphic sites segregating within the PanIA allelic class. The differentiation observed at silent sites in the PanIA allelic class contradicts the predicted effects of widescale gene flow and suggests that postsettlement selection acting on cohorts cannot be responsible for the genetic differences described between coastal and Arctic populations. Our results suggest that the marked differences observed between coastal and Arctic populations of G. morhua in northern Norway at the PanI locus reflect the action of recent diversifying selection and that populations throughout the region may be more independent than suggested by previous studies.     

Lethal oxygen levels at different temperatures and the preferred temperature during hypoxia of the Atlantic cod, Gadus morhua L.

The behavioural thermoregulation of Atlantic cod Gadus morhua L. was investigated in a shuttlebox at normoxia and at three levels of hypoxia: 30, 20 and 15% oxygen saturation.
The preferred temperatures at normoxia, 30, 20 and 15% oxygen saturation were 13·9, 13·1. 10·0 and 8·8° C, respectively.
A decrease in metabolism and an increased blood oxygen affinity are among the physiological advantages of selecting a lower temperature during hypoxia. Furthermore the chances of surviving low oxygen saturations are better at low temperatures.
In natural environments, this behaviour may result in habitat shifts of fish living in heterothermal environments with changing oxygen saturations, especially in coastal areas with eutrophication, as for example the Baltic Sea.     

Temperature-dependent changes in energy metabolism, intracellular pH and blood oxygen tension in the Atlantic cod

The effect of acute increase in temperature on oxygen partial pressure (Po ₂) was measured in the gill arches of Atlantic cod Gadus morhua between 10 and 19° C by use of oxygen microoptodes. Oxygen saturation of the gill blood under control conditions varied between 90 and 15% reflecting a variable percentage of arterial or venous blood in accordance with the position of each optode in the gill arch. The data obtained suggested that arterial Po₂ remained more or less constant and arterial oxygen uptake did not become limiting during warming. A progressive drop in venous Po₂, however, was observed at >10° C indicating that excessive oxygen uptake from the blood is not fully compensated for by circulatory performance, until finally, Po₂ levels fully collapse. In a second set of experiments energy and acid–base status of white muscle of Atlantic cod in vivo was measured by magnetic resonance (³¹P‐NMR) spectroscopy in unanaesthetized and unimmobilized fish in the temperature range between 13 and 21° C. A decrease in white muscle intracellular pH (pH_i) with temperature occurred between 10 and 16° C (ΔpH per ° C = ?0·025 per ° C). In white muscle temperature changes had no influence on high‐energy phosphates such as phosphocreatine (PCr) or ATP except during exposure to high critical temperatures (>16° C), indicating that white muscle energy status appears to be relatively insensitive to thermal stress if compared to the thermal sensitivity of the whole animal. The data were consistent with the hypothesis of an oxygen limitation of thermal tolerance in animals, which is set by limited capacity of oxygen supply mechanisms. In the case of Atlantic cod circulatory rather than ventilatory performance may be the first process to cause oxygen deficiency during heat stress.