Temporal, spatial and tidal influences on benthic and demersal fish abundance in the Forth estuary

The 10 most common demersal and benthic fish species collected during a 1982–2001 Agassiz trawling programme in the mid/ lower Forth estuary, east Scotland, were assessed for possible influences of trawl site, month of sampling and tide height on abundance. All species were seasonal in their occurrence in the study area, with trends generally similar to those shown in other temperate European estuaries. Six species (whiting (Merlangius merlangus), dab (Limanda limanda), pogge (Agonus cataphractus), fatherlasher (Myoxocephalus scorpius), gobies (Pomatoschistus spp.) and sea snail (Liparis liparis)) exhibited high winter and low summer abundance while plaice (Pleuronectes platessa), flounder (Platichthys flesus) and eelpout (Zoarces viviparous) were least numerous in winter and mostly present in summer. Cod (Gadus morhua) were low in abundance in late spring, but did not show pronounced peaks of abundance. Trawl station significantly influenced fish abundance in all but one case ( gobies ). There was relatively low abundance of most species at the middle trawl location, Tancred; only plaice, dab and whiting were numerous at this site. The majority of species were abundant at Port Edgar, adjacent to the estuary mouth, which may have reflected the relatively stable salinity conditions at this station. Flounder was most abundant at the mid-estuarine site of Longannet, and this was likely to have been in order to feed on the large mudflats nearby. Dispersal of fish onto inundated intertidal areas is suggested as a possible mechanism explaining the significant decrease in abundance of several species (flounder, pogge, eelpout and gobies ) at high water (HW). The significant increase in abundance of whiting in the study area at HW may have been attributable to immigration of individuals from inshore regions of the Firth of Forth by the action of the flood tide. Interactions of location, month and tide height significantly influenced abundance of most species captured during trawling, though explained less variation in the data than the individual main effects.