Lars-Ove Eriksson

The status of wild and hatchery propagated Swedish salmon stocks after 40 years of hatchery releases in the Baltic rivers

Abstract The Baltic stocks of Atlantic salmon have been subjected to human exploitation for many centuries. During the last century, however, these salmon populations have been subjected to especially dramatic changes in spawning stream access, fishing patterns, and fishing pressure. At the end of the nineteenth century, salmon spawned in 60–70 rivers in the Baltic proper. From 1940 onwards many of the large salmon rivers were dammed, most of them below the lowest spawning rapids for salmon. Methods for rearing salmon up to the smolt stage were developed, and the first smolt releases started around 1950. The number of smolts released in Swedish rivers gradually increased to about 2–2.5 million by the middle of the 1980s. During the last 10 years other countries have started salmon release programs, resulting in an annual release of about 3 million smolts. Smolt releases have been successful, showing high survival rates of stocked fish. Today, only about 20 suitable rivers, most of them in Sweden, are available for natural spawning runs. The runs are very weak. Before the Second World War the Baltic salmon fishery mainly consisted of coastal and river catches of spawning migrators ascending the rivers. However, since the end of the war an offshore drift gillnet fishery has been developed, and nowadays makes up the major part of the total catch. As a consequence, the spawning runs of wild salmon have decreased dramatically. One hundred years ago the natural production of salmon from the Baltic salmon rivers was around 7–8 million smolts annually. In the early 1970s, the wild smolt run to the Baltic included about 2 million fish. At present, only about 0.5 million wild smolts are recruited to the whole Baltic proper, which is only about 20% of the smolt production capacity still remaining in the rivers. The offshore fishery on mixed populations in the central areas of the Baltic proper leads to an extremely high fishing pressure on the Baltic salmon stocks. Wild stocks are unable to cope with the present exploitation rate. As a result, the number of wild spawners in natural streams has gradually been diminishing. In the absence of a compensatory program, a reduction in stock-size of the magnitude shown by wild salmon in the Baltic would decrease catch-per-unit-effort to such a degree that there would be no economic incentive for a commercial salmon fishery. At present, however, about 90% of the salmon smolts leaving Baltic rivers are of hatchery origin. Owing to their extremely high egg-to-smolt survival rates the reared stocks can withstand this high exploitation. Thus the success of hatchery enhancement programs in combination with a lack of effective regulations, allowing a high-catch-per unit effort in the offshore fishery, is largely responsible for the sad fate of remaining wild salmon stocks in the Baltic. Any biologically sound management program for the Baltic salmon would have to include regulations prohibiting or severely restricting the offshore fishery on mixed stocks. Salmon stocks should be exploited in relation to the capacity of individual populations, by a more terminal fishery at the coast or in rivers. However, an established offshore fishery in the main Baltic, including (at least) seven nations, has so far made any kind of regulation difficult.

The Swedish Arctic charr breeding programme

Results and experiences from a selective breeding programme aiming at improving the performance of an Arctic charr strain in aquaculture are presented. The programme, which has been running since 1985, uses traditional quantitative genetic methods based on relatedness and trait measurements. Traits considered included growth, age at sexual maturity, flesh colour, fat content and other features. Estimates in the early phase of the programme showed promising heritabilities for most traits, e.g. growth heritabilities of 0.34–0.52, and suggested that improvement by selection was feasible. Several traits such as a high rate of early maturation and poor flesh colour that were considered problematic by the Arctic charr farming industry at the programme’s start no longer hamper farming. Considerable improvement of growth by selection, estimated at 8% per generation, has contributed to shortening the production cycle in commercial farming. Results from studies of genotype–environment interactions are presented and discussed. Poor survival of fertilised eggs is a major problem in Swedish Arctic charr farming as indicated by presented survival rates. Efforts have consistently been made to avoid inbreeding in the selected strain, and at the present seventh generation the accumulated increase in inbreeding is estimated from pedigree data to be only approximately 5%.

Potential toxic effect on aquatic fauna by the dwarf shrub : Empetrum hermaphroditum

The common evergreen dwarf shrub Empetrum hermaphroditum has influence on the functioning of boreal terrestrial ecosystems in northern Sweden. The negative effects of E. hermaphroditum are partly attributed to the production of the dihydrostilbene, batatasin-III, which is released from leaves and litter by rain and snowmelt. In this study, we investigated whether batatasin-III is carried by runoff into streams and lakes during the snowmelt period and whether it is also potentially hazardous to aquatic fauna. Sampling of water from streams and a lake for which the surrounding terrestrial vegetation is dominated by E. hermaphroditum was done during the snowmelt period in May 1993 and in 1998, and analyzed for batatasin-III. Using 24- and 48-hr standard toxicity tests, we analyzed toxicity to brown trout (Salmo trutta) alevins and juvenile water fleas (Daphnia magna). Toxicity (proportion of dead individuals) to trout was tested at pH 6.5 and compared with that of a phenol within a range of concentrations. In the toxicity (proportion of immobilized individuals) test on D. magna, the interactive effect of pH (pH 5.5–7.0) was included. Concentration of batatasin-III was generally higher in 1998 than in 1993 and showed peak levels during snowmelt. Concentration in ephemeral runnels > the lake > streams running through clear-cuts dominated by E. hermaphroditum > control streams lacking adjacent E. hermaphroditum vegetation. The maximum concentration of batatasin-III found was 1.06 mg l−1. The proportion of dead yolk sac alevins increased significantly (P < 0.001) with increasing concentrations of batatasin-III and time of exposure. After 24 hr, EC50 was 10 mg l−1. It was 2 mg l−1 after 48 hr. The effect of phenol was negligible, indicating a specific phytotoxic effect of the bibenzyl structure of batatasin-III. The proportion of mobile D. magna became significantly smaller (P < 0.001) with increasing concentrations of batatasin-III, with decreasing pH, and with increasing exposure time. EC50 varied between 7 and 17 mg l−1 at pH 5.5 and 7.0, respectively. After 24 hr EC50 decreased and was 2.5 at pH 5.5 and 12 mg l−1 at pH 7.0. The levels of batatasin-III found in the field samples were below the lowest EC50 in acute toxicity tests. However, in view of the interactive effect of pH and exposure time, this study suggests that this stable plant metabolite may impose a lethal effect on the aquatic fauna in small streams.