A Bayesian estimate of harbour seal survival using sparse photo-identification data

Survival rates have rarely been estimated for pinniped populations due to the constraints of obtaining unbiased sample data. In this paper, we present an approach for estimating survival probabilities from individual recognition data in the form of photographic documentation of pelage patterns. This method was applied to estimate adult (age 2+) survival for harbour seals in the Moray Firth, NE Scotland. An astronomical telescope was used to obtain digital images of individual seals, and high-quality images were used to document the annual presence or absence of individuals at a single haul-out site over a 4-year period. A total of 95 females, 10 males and 57 individuals of unknown sex were photographically documented during the study period. Survival and recapture probabilities were estimated using Jolly–Seber mark–recapture models in a Bayesian statistical framework. Computer-intensive Markov Chain Monte Carlo methods were used to estimate the probability distributions for the survival and recapture probabilities, conveying the full extent of the uncertainty resulting from unavoidably sparse observational data. The deviance information criterion was used to identify a best-fitting model that accounted for variation in the probability of capture between sexes, with constant survival. The model estimated adult survival as 0.98 (95% probability interval of 0.94–1.00) using our photo-identification data alone, and 0.97 (0.92–0.99) with the use of an informative prior distribution based on previously published estimates of harbour seal survival. This paper represents the first survival estimate for harbour seals in the UK, and the first survival estimate using photo-identification data in any species of pinniped.     

Seasonal Timing and Diel Activity of Lacustrine Brook Charr, Salvelinus Fontinalis, Spawning in a Lake Outlet

The objectives of this study were to determine (1) the seasonal timing and diel activity patterns of lacustrine brook charr when migrating to a lake outlet for spawning, and (2) the homing of reproducing individuals. In 1995, 745 individuals were captured while migrating to the spawning ground compared to 1148 in 1996. In both years, the sex ratio was not significantly different from 1:1 but females were significantly longer than males. The spawning migrations lasted about 48d and were more nocturnal (between 20:00 and 8:00h) than diurnal. The migration to the spawning ground began when water temperature decreased to 13°C and peaked when water temperature decreased from 11°C to 7°C. Peaks in migration activity were always preceded by a sudden decrease followed by an increase in temperature. The spawning migration lasted until the third week of October, when water temperature varied between 3°C and 8°C. Males, especially larger ones, arrived on the spawning ground before females in both years. The length of males migrating to the spawning ground decreased during the spawning season while the females' length showed no pattern. Only 9.7% of the 745 fin-clipped individuals from 1995 returned to the lake outlet to spawn in 1996. The results of this study indicate that lacustrine brook charr show similarities to other anadromous and lake-spawning salmonid populations when migrating to spawning grounds.     

Sediments,not plants,offer the preferred refuge for Daphnia against fish predation in Mediterranean shallow lakes: an experimental demonstration

1. Different behavioural responses of planktonic animals to their main predators, fish, have been reported from shallow lakes. In north temperate lakes, large‐bodied zooplankton may seek refuge from predation among macrophytes, whereas in subtropical lakes, avoidance of macrophytes has been observed. The prevalent behaviour probably depends on the characteristics of the fish community, which in Mediterranean lakes is typically dispersed in both the open water zone and in the littoral, as in temperate lakes, and is dominated by small size classes, as in subtropical lakes. 2. We performed ‘habitat choice’ experiments to test the response of Daphnia magna to predation cues at both the horizontal and vertical level by mimicking a ‘shallow littoral’ zone with plants and a ‘deeper pelagic’ zone with sediments. 3. Initial separate response experiments showed that natural plants, artificial plants and predation cues all repelled D. magna in the absence of other stimuli, while sediments alone did not trigger any significant response by D. magna. 4. The habitat choice experiments showed that, in the presence of predation cues and absence of plants, Daphnia moved towards areas with sediment. In the presence of both plants and sediments, Daphnia moved away from the plants towards the sediments under both shallow and deep water treatment conditions. 5. Based on these results, we suggest that Daphnia in Mediterranean shallow lakes avoid submerged macrophytes and instead prefer to hide near the sediment when exposed to predation risk, as also observed in subtropical shallow lakes. This pattern is not likely to change with water level alterations, a common feature of lakes in the region, even if the effectiveness of the refuge may be reduced.     

Migratory behaviour of adult pikeperch (Stizostedion lucioperca) in a lowland river

The behaviour of radio-tagged adult pikeperch (Stizostedion lucioperca (L.)) from two areas in the Danish River Gudenaa were recorded prior to, and during the spawning period. Eight of 13 tagged fish in the lower reaches of the river were located throughout the whole study. Five of these fish moved upstream to various sites in the river prior to spawning, which occurred from late April to June. The three remaining fish moved to the fjord. These movements were interpreted as a spawning migration, and it is suggested that the lower reaches of the River Gudenaa constitute an over-wintering area for pikeperch, which use different spawning areas. Ten pikeperch caught just downstream of an impassable hydropower plant were radio-tagged and translocated upstream of the dam to a reservoir. Within a week, half of the fish moved to a lake situated more than 30 km upstream the reservoir. This behaviour is hypothesised to be a homing response. The study reveals that the pikeperch is a highly mobile species with a complicated migration pattern, even in relatively small river systems.