The quality of the surface water of Gotland (Sweden) streams was good, and did not limit sea trout egg-to-fry (ETF) survival. The oxygen concentration of the interstitial water was positively correlated to the permeability of the stream bed, and to the geometric mean diameter of the substratum. When the oxygen at 15 cm inside the stream bed was undiminished from surface values, the permeability of the stream bed was at least 6000 cm h 1 and the geometric mean diameter of the substratum at least 15·0 mm. When the interstitial oxygen concentration increased, the interstitial ammonium concentration decreased, the NO 2 concentration remained stable and the NO 3 concentration increased. The ETF survival up to hatching was highly dependent upon the oxygen concentration of the interstitial water. For survival from hatching to emergence, the geometric mean diameter of the substratum was the most important factor. ETF survival >50% was observed where the interstitial oxygen concentration averaged at least 10 mg l 1 , the substratum permeability at least 2000 cm h 1 and the substratum geometric mean diameter at least 15 mm. Other factors, such as overdigging and drying out of the spawning grounds could increase the ETF mortality. 1996 The Fisheries Society of the British Isles

Characteristics of the natural spawning habitat of sea trout, Salmo trutta L., were studied in Sja¨lso¨a˚n, a small stream of Gotland, Sweden, from 1992 to 1999. Each year, trout spawned on 17 ± 7 different areas (156 places ha )1 ). Most of the spawning grounds were used every year. Overcutting was evident for at least 60% of the spawning grounds. Fish spawned on a gravel of 19 ± 7 mm in diameter.From 1978 to 1992, 242 artificial spawning grounds were constructed by the Gotland Sport Fishermen Association in four Gotland streams. A sediment trap was dug upstream to the spawning grounds. The artificial spawning ground comprised of a downstream V-shape stream deflector of large stones with a log weir at the narrowest point of the deflector. Upstream of the dam, 15-60 mm diameter gravel was deposited to constitute the spawning ground substratum. To keep the installation efficient, maintenance is needed every year before the spawning season. K E Y W O R D S : natural and artificial spawning grounds, Salmo trutta, Sweden.

The spawning pattern of the anadromous brown trout Salmo trutta was studied in Sja¨lso¨a˚n, a small stream in Gotland, Sweden, during eight winters between 1992-1993-2000. 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 (2þ to 4þ years) than females (3þ to 5þ years). The sex-ratio at the beginning and at the end of the spawning season favoured males. The mean AE S.D. number of spawners was 70 AE 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 (29Á3% of all the females) or the next night (32Á8% of all the females) while males may have stayed for 2 to 3 weeks (21Á3% of all the males) in the stream before spawning. Males usually remained much longer in the stream (mean AE S.D. 45 AE 56 days) than females (16 AE 30 days). Females lost more mass in the stream (mean AE S.D. 17Á3 AE 8Á6%) than males (7Á7 AE 9Á6%). For both sexes, mass loss was positively correlated with the time spent in the stream. Only 7Á3% of the males and 5Á7% of the females occurred in the stream for >1 year. Spawning took place only during the night. # 2005 The Fisheries Society of the British Isles

In a study of the genetic relationships among 879 anadromous brown trout Salmo trutta from 13 streams at the Island of Gotland, Sweden, using RFLP analysis of a mitochondrial DNA segment (NADH dehydrogenase-1 gene), six haplotypes were detected. Significant genetic divergence was observed among streams as well as between cohorts within streams. Approximately 8-9% of the total variation was due to differences between populations, and 4-5% was explained by differences between cohorts within populations. The female effective population size (N ef ) was assessed from temporal haplotype frequency differences between consecutive cohorts; the estimated average N ef over all populations was just below 30, suggesting that these populations were effectively quite small. With such small effective sizes the populations are expected to loose genetic variability quickly, but the observed levels do not appear particularly low. This indicates that female migration between streams occurs. The observed level of differentiation does not support the presumption that a particular pre-smolt migratory behaviour observed in Gotland streams, with large portions of fry leaving for the sea soon after hatching, results in a reduced homing ability. From a conservation management perspective the Gotland brown trout streams should be regarded as a population system where the vitality and survival of brown trout in one stream is dependent on the opportunity of contact and exchange of individuals from other streams. 2002 The Fisheries Society of the British Isles