The effects of flow on Atlantic salmon (Salmo salar) redd distribution in a UK chalk stream between 1980 and 2015

Atlantic salmon are an ecologically and economically important migratory fish in the UK, whose stocks have been declining over the past 30 years. Future climate and water use changes have the potential to alter the reproductive behaviour and distribution of salmon within a river, by restricting times and ability to access suitable spawning areas. As the survival of emergent salmon juveniles is density dependent, understanding how climate‐driven changes in flow affect the location of salmon redds is important for future conservation efforts. This study examined how flow conditions affect the distribution of redds within a UK chalk stream, the river Frome in Dorset. Sixteen years of redd distribution and flow data between 1980 and 2015 were analysed using linear mixed‐effects modelling. Generally, highest redd densities occurred within middle reaches of the main river. Mean flow during the river Frome critical migration period (October–December) did not affect the density of redds directly but affected the relationship between redd density and distance from tidal limit: redd densities were spread more uniformly throughout the river under high flow conditions, whereas redds were more aggregated in the middle river reaches under low flow conditions. Together, these findings suggest that access to upstream spawning grounds was limited under low flow conditions, which could have negative repercussions on juvenile survival. This study has revealed the distribution of redds along the river Frome for the first time and provided a basis for further study into the effects of redd distribution on subsequent juvenile life stages.     

Importance of the River Teviot to Atlantic salmon, Salmo salar, rod catches in the River Tweed, Scotland

Abstract  The body mass distributions and variations in abundance of the spring and autumn catches of Atlantic salmon, Salmo salar L., were compared between fisheries above and below the confluence of the rivers Tweed and Teviot, Scotland. Significant differences between the distributions were found over three periods, one in spring and two in autumn, when salmon catches were high. Correlations were found between abundance trends for periods when distributions were similar or dissimilar. Abundance of salmon returning to the River Teviot was proportional to the main river. However, its population structure differed when abundances were high, suggesting different survival strategies. It appears that changes in the numbers returning to the river were resultant of changes in the marine rather than freshwater environment.     

Uncertainty about estimating total returns of Atlantic salmon, Salmo salar to the Gander River, Newfoundland, Canada, evaluated using a fish counting fence

Abstract  For a number of rivers in Newfoundland, Atlantic salmon, Salmo salar L., is managed in relation to river-specific conservation spawning requirements. One such river is the Gander River, where between 1989 and 1999, the escapement of Atlantic salmon, a major factor in assessing the status of stock, was determined using a fish counting fence. In 2000, the counting fence was discontinued and alternative means of calculating total returns were explored. Regression and simulation methods, using relationships between total returns and salmon counts at an upstream tributary during 1989–99, formed the basis for estimates of returns for 2000, and the uncertainty around estimates. The accuracy of methods is evaluated by retrospective comparisons with actual total returns between 1989 and 1999. Estimates of total returns deviated from the actual by as much as 50–60%, depending on the method. Management implications of the approach are discussed.     

Differences in the time of river entry of Atlantic salmon,Salmo salar L., spawning in different parts of the River North Esk

Abstract Spawning adult Atlantic salmon, Salmo salar L., were sampled in five tributaries of the River North Esk, Scotland, during the winters 1989/90 and 1990/91. The fish furthest upriver had higher sea- and smolt-ages and the one-sea-winter fish were smaller and more coloured than those spawning further downstream. These differences indicate that the uppermost tributaries are populated by fish which enter the river early in the year and the lowest tributaries by fish entering late in the year. Exactly why such a structure exists is not clear, but it is suggested that these differences may be adaptive and that selection effects may continually be modifying these populations.     

Survival of reared Atlantic salmon and sea trout in relation to marine conditions of smolt year in the Baltic Sea

The marine survival of hatchery-reared Atlantic salmon (Salmo salar) and sea trout (Salmo trutta) was examined in relation to marine conditions during post-smolt migration and in relation to stock traits. In 1970–2001, Carlin-tagged smolts were released in the Iijoki and Oulujoki rivers, the northern Baltic Sea. When both species were analysed together, the abundance of the three prey fish, herring (Clupea harengus), smelt (Osmerus eperlanus) and vendace (Coregonus albula) correlated positively with the survival of salmonids. In addition, the increase in smolt size appeared to improve the survival rate. Sea surface temperature (SST) may have affected indirectly through the abundance of prey fish during the post-smolt migration of salmon and sea trout. The smelt and vendace showed a statistical effect on survival only when the temperature effects were not included in the models. In sea trout, an increasing smolt length was not significantly correlated with the survival in good herring recruitment years, but in poor years survival increased very rapidly with increasing smolt size. The recapture rates of the salmonids tended to decrease between the years 1970 and 2001. During the same time period, the June SST slightly decreased. The positive correlation between the annual summer SST and recapture rate of salmon may partly explain the decreasing trend in recapture rates. An increase in smolt size did not compensate for the decline in the recapture rate of either species.     

Comparison of population dynamics of Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) in a small tributary of the River Scorff (Brittany, France)

The population dynamics of Atlantic salmon (Salmo salar L.) and brown trout (Salmo trutta L.) were compared in a small tributary of the River Scorff (Brittany, France) from spawning time to the beginning of the third growing season. The spawning and fry emergence of the two species took place at approximately the same time. In the first autumn, the densityof 0+ juveniles and settling rate from the egg stage were much higher in trout than in salmon. The emigration rate from 0+ population was much higher in trout than in salmon. The size of resident and migrating fish was always smaller in salmon than in trout, whatever the age. The low level of salmon production in the brook, compared with trout, was the result of low survival from egg to 0+ stage in autumn, combined with the small proportion of juveniles migrating after the first growing season. This was not compensated by a high number of migrants the next year. The role of physical habitat, inter-and intraspecific competition, predation and migration dependence on size and early sexual maturity is discussed. Indications are that small tributaries of the type studied are of great value for recruitment in trout but not very productive for juvenile salmon.     

Timing of migration and changes in age structure of Atlantic salmon, Salmo salar L., in the River Frome, a Dorset chalk stream, over a 24-year period

During the last 24 years, the number of Atlantic salmon, Salmo salar L., migrating up the River Frome has varied between < 1000 and > 4000 fish per annum. Most parr become smolts at age 1+ and there are three main sea ages of salmon returning to the river. Each sea age shows a bimodal pattern of migration in the river. Primary migration times for 3-sea-winter (3SW) fish are spring and autumn, with 2SW fish and grilse (1SW) peaking in summer and autumn. Some 28.4% of 3SW and 6.9% of 2SW fish are previous spawners. Multi-sea-winter fish historically passed over the counter in all months of the year, but recently, their numbers have declined, as have total numbers of fish of all ages. There has been a fundamental change in the age structure of the population, with grilse increasing in proportion over the study period. Photographs of fish, taken for validation of the counter, showed a significant decrease in mean length of grilse, but an increase in mean length for 2SW and 3SW fish. Rod catch data support these findings.     

Historical changes in age structure of Atlantic salmon, Salmo salar L., in the River Eo, northern Spain

Abstract Variations in the age structure and number of Atlantic salmon, Salmo salar L., running into the River Eo in northern Spain were assessed from catch records of sport fisheries for the period 1949 to 1991. The analysis focused on two periods (1951–1960 and 1981–1991) for which more complete data on fish number, size and age were available. Over the whole period, no evidence for a decline in the total salmon run was found, but the last few years revealed significant changes in both the number and age structure of salmon caught. The catches were mainly composed of multi-sea-winter salmon, and this did not change from the 1950s to the 1980s as the proportion of grilse remained unchanged. However, since 1980, there was a marked decline in both the proportion of 3-sea-winter salmon (versus 2-sea-winter fish) and the frequency of previous spawners among the multi-sea-winter salmon. These changes were probably linked because most of the previous spawners re-entered the river as 3-sea-winter or older salmon. River age of salmon caught in the 1950s was low and constant (about 90% of the catch had smolted at one year) but the mean age of smolting increased in the 1980s.     

Dorsal aorta cannulation: a method to monitor changes in blood levels of astaxanthin in voluntarily feeding Atlantic salmon, Salmo salar L.

This study was undertaken to assess dorsal aorta cannulation as a method to evaluate alterations in diet composition and feeding protocol on pigment retention in salmonid fish. Temporal changes in blood astaxanthin concentrations of dorsal aortacannulated Atlantic salmon, Salmo salar L., were followed in relation to variations in dietary pigment concentration and fish-feeding husbandry protocol. The fish were held individually in 200-L fibreglass tanks supplied with running sea water. Each fish was forced to swim at 0.5 body lengths s^?1 and was fed daily by hand to satiation. The fish had an average growth rate of 1% day^?1. Blood astaxanthin concentrations were noted to be highly correlated (r= 0.995) with dietary levels of astaxanthin, but not as well correlated (r= 0.71) with total gut content of this pigment. Marked variations in blood astaxanthin concentration were noted between individual fish at each dietary pigment concentration, but the ranking of the fish was generally unaffected between each dietary pigment level. After cessation of feeding a diet supplemented with 75 mg of astaxanthin kg^?1, salmon fed a diet with no pigment showed more-rapid blood pigment clearance than those that were starved. Likely, feed remaining in the alimentary tract of the starved fish functioned as a reservoir of pigment for the blood until the intestinal tract was empty. Blood pigment levels were not depressed in salmon fed a diet supplemented with 75 mg of astaxanthin kg^?1 once daily instead of twice daily.     

Growth and feed utilization of large size rainbow trout (Oncorhynchus mykiss) and Atlantic salmon (Salmo salar) reared in freshwater: diet and species effects, and responses over time

Four diets differing in crude protein/crude lipid concentrations (CP/CL), 570/200, 510/220, 460/240, 430/260 (g kg^?1 dry diet) were fed to near‐satiety to rainbow trout (initial body weight, IBW = 268 g, domestic strain) and Atlantic salmon (IBW = 456 g, wild strain) for 308 days to determine the effect of diets, species and fish size on efficiency of feed, nitrogen (N) and energy utilization. Within species, weight gain, feed efficiency (FE), and energy retention efficiency (ERE, E gain/E intake) were not affected by diet (P < 0.05). N retention efficiency (NRE, N gain/N intake) increased linearly (P < 0.05) with decreasing CP/CL for both species. NRE and FE were higher for salmon compared with trout. Whole body CP of salmon was significantly (P < 0.05) higher, and lipid significantly (P < 0.05) lower, than those of trout. There was a significant (P < 0.05) linear decrease in FE as fish grew, regardless of diet and species. NRE linearly decreased (P < 0.0001) and lipid to protein deposition ratio (LD/PD) increased (P < 0.05) as trout grew. NRE linearly increased (P < 0.05) while ERE and LD/PD linearly decreased (P < 0.05) as salmon grew. Salmon appeared to rely to a greater extent on non‐protein energy‐yielding nutrients to support cost of growth than did trout.     

Dietary fatty acids and inflammation in the vertebral column of Atlantic salmon, Salmo salar L., smolts: a possible link to spinal deformities

Vegetable oils (Vo) are an alternative to fish oil (Fo) in aquaculture feeds. This study aimed to evaluate the effect of dietary soybean oil (Vo diet), rich in linoleic acid, and of dietary fish oil (Fo diet) on the development of spinal deformities under bacterial lipopolysaccharide (LPS)‐induced chronic inflammation conditions in Atlantic salmon, Salmo salar L. Fish [25 g body weight (BW)] were fed the experimental diets for 99 days. On day 47 of feeding (40 g BW), fish were subjected to four experimental regimes: (i) intramuscular injections with LPS, (ii) sham‐injected phosphate‐buffered saline (PBS), (iii) intraperitoneally injected commercial oil adjuvant vaccine, or (iv) no treatment. The fish continued under a common feeding regime in sea water for 165 more days. Body weight was temporarily higher in the Vo group than in the Fo group prior to immunization and was also affected by the type of immunization. At the end of the trial, no differences were seen between the dietary groups. The overall prevalence of spinal deformities was approximately 14% at the end of the experiment. The Vo diet affected vertebral shape but did not induce spinal deformities. In groups injected with LPS and PBS, spinal deformities ranged between 21% and 38%, diet independent. Deformed vertebrae were located at or in proximity to the injection point. Assessment of inflammatory markers revealed high levels of plasma prostaglandin E₂ (PGE₂) in the Vo‐fed and LPS‐injected groups, suggesting an inflammatory response to LPS. Cyclooxigenase 2 (COX‐2) mRNA expression in bone was higher in fish fed Fo compared to Vo‐fed fish. Gene expression of immunoglobulin M (IgM) was up‐regulated in bone of all LPS‐injected groups irrespective of dietary oil. In conclusion, the study suggests that Vo is not a risk factor for the development of inflammation‐related spinal deformities. At the same time, we found evidence that localized injection‐related processes could trigger the development of vertebral body malformations.