Young-of-the-year fish species composition in small coastal bays in the northern Baltic Sea, surveyed with beach seine and small underwater detonations

The young-of-the-year (YOY) fish species composition of 12 small coastal bays in the Tvärminne region, southwestern Finland, was surveyed in late summer 2003 using a beach seine and underwater detonations. The surface area of the bays varied between 1 and 21 ha, and they were located along a clear exposure gradient reaching from an innermost bay system, where the surface water salinity was 1%–3%, to an outer archipelago, where salinity was around 5%–6%. The beach seine data demonstrated a change in dominance from freshwater species in the inner areas to marine species in the more exposed outer areas. Using the beach seine, the average number of YOY fish species in a single bay was 6.2 and the total number of YOY fish species caught was 17, while only four species were caught by detonation. Detonation sampling caught effectively only the YOY of some early spawning freshwater species, such as perch and roach. For these two species, both methods gave quite similar abundance patterns in the study bays. The YOY of some other common species, such as bleak and sticklebacks, were too small (8–22 mm) to be caught with the detonation method. The results demonstrated a rather low sampling precision with both methods even for the most abundant freshwater species, the main reason being that YOY fish are usually highly aggregated leading to a high frequency of zero samples.