The effectiveness of different gonadotropin releasing hormone analogues (GnRHa) and/or pimozide for inducing ovulation in Arctic charr was investigated at two different temperatures: 5 °C, a temperature suitable for spontaneous ovulation in Arctic charr and 10 °C, a temperature which inhibited ovulation in Arctic charr. At 5 °C all the different GnRH analogues tested were able to induce and synchronize ovulation. At 10 °C a sustained release preparation of D-tryptophan6 luteinizing hormone releasing hormone (D-Trp6LH-RH) at 20 μg.kg−1 and an acute release preparation of D-arginine6salmon GnRH (D-Arg6 sGnRH) at 100 μg.kg−1 were able to induce ovulation in 80% of the females within 15 days (versus 2% in controls). Pimozide alone or in combination with a low dose of D-Arg6sGnRH was able to induce some ovulation at 10 °C, suggesting that a dopamine inhibition of gonadotropin secretion could occur at 10 °C. Egg survival in groups receiving GnRHa at 5 °C was comparable to controls (73%) except in the group receiving GnRHa in combination with pimozide (45%). At 10 °C, egg survival was significantly lower than controls at 5 °C, except for the group receiving pimozide alone. At both temperatures, egg survivals of each spawn were negatively correlated with the plasma gonadotropin (GtH2) level of the reproducing females except in groups receiving a sustained release preparation of D-Trp6LH-RH.

The objectives were to evaluate temporal variation in 137Cs in trout and charr from Crummock Water, Wastwater, Ennerdale Water, and to compare levels in trout from six lakes (above three + Windermere, Loweswater, Devoke Water) and charr from four lakes (above three + Windermere). Fish were caught between June 1986 and October 1988, chiefly with fyke nets and gill nets. Overall means for 137Cs in trout were markedly higher than those for charr in the three lakes. Values for individual trout exceeded 1000 Bq.kg-1 in Ennerdale and 500 Bq.kg-1 in Wastwater and Crummock Water, but values for charr never exceeded 350 Bq.kg-1. 137Cs content increased with fish weight for trout from Wastwater and therefore values were scaled to a standard weight in subsequent analyses. No similar relationships were found for trout in Ennerdale or Crummock Water, or for charr in all three lakes. Monthly mean values for 137Cs in trout followed similar temporal patterns in the three lakes and a parabola provided a simple model for these changes. Chernobyl deposition occurred early in May 1986 and maximum values in trout were in December 1986 in Wastwater, January/February 1987 in Ennerdale Water and March 1987 in Crummock Water. The exponential rate of decrease from these maxima did not differ significantly between the three lakes, and ecological half-lives of 137Cs in the trout differed slightly, but not significantly, between lakes : 180 days for Wastwater, 194 days for Crummock Water and 249 days for Ennerdale Water. The equivalent value for Ennerdale charr was lower at 132 days, and it was impossible to fit models or calculate half-lives for Wastwater and Crummock charr. Maximum monthly geometric mean 137Cs values in trout from six lakes and charr from four lakes were related to the initial concentration of 137Cs in both water and sediment, and the maximum monthly geometric mean 137Cs values obtained from the routine water samples. The relationship was curvilinear and approximated, but was not identical to, an asymptotic curve. No similar relationships were found with mean Ca or K levels in the lake water. Metabolic and diet differences may explain the lower 137Cs levels in charr compared with trout and differences for the same species from different lakes may be due to limnological variations and différences between lakes in the nature of their sediments and catchment soils. Both points are discussed in detail.

All English populations of Arctic charr are found in the Lake District (northwest England). There are at least four races of charr in Windermere, the largest natural lake in England, with the north and south basins of the lake each containing two distinct races that spawn in autumn and spring respectively. Three methods have been used to estimate levels of the charr stocks ; namely gill-netting each November (1939-1991) for autumn spawners in the north basin, anglers' catches from both basins (1966-1991), echo-survey data from both basins (July 1989 - December 1991). An increase in gill-net catches from 1945 to 1965 was associated with a cull of larger pike, Esox lucius L. ; since 1965, catches have fluctuated around a high mean level. There was good, if not excellent, agreement between the anglers' catches and the gill-net catches. The ratio of anglers' catches in the north and south basins showed that catches have been relatively low in the south basin since 1984. Although there has also been an increase in brown trout, Salmo trutta L., taken by the charr anglers since 1984 in the south basin, this increase was not responsible for the lower charr catches. When estimates of fish densities with an echo sounder were restricted to fish > 20 cm and depths down to 20 m, this being roughly the stock available to charr anglers, there was good agreement between monthly estimates and anglers' catches, especially in the north basin. The echo-sounder data confirmed the higher numbers in the north basin, and lower numbers in winter in both basins. The three different methods therefore provided comparable data on the fluctuations in levels of charr stocks in Windermere. Possible reasons for these fluctuations are discussed briefly.

Development of a broodstock of Arctic charr was undertaken in experimental tanks supplied with water pumped from Lake Léman (Geneva) at a depth of 36 metres. Spawners were subjected todifferent thermal regimes to determine the effects of temperature on spawning time and gameteviability. In the tanks, the ovulation occurred spontaneously at the same time as the wild fish in the lake. The timing of ovulation was slowed down at 8 °C and above. At 11 °C, ovulation was inhibited. When the water temperature of the rearing tanks was higher than 5 °C for several weeks prior to spawning, the quality of eggs produced by reared females was poorer than those of wild fish because the process of overripening was very rapid above 5 °C. Provided that spawners were reared at 5 °C and examined twice a week to detect ovulation, 80 to 90 percent survival to the eyed stage could be expected for eggs of reared females. The fertilization rate of eggs produced by spawners transferred from 8 to 5 °C in December was high (78%). When the transfer occurred in January, the viability of ova tended to decrease (63%).