Population dynamics, biology and ecology of the caridean shrimps : Crangon crangon Linnaeus, Crangon allmanni Kinahan and Pandalus montagui leach in the estuary and Firth of Forth, Scotland
The population of shrimps from five stations in the estuary and one station in Firth of Forth were sampled for two years from January 1992 to investigate their population dynamics, reproductive biology and feeding ecology. Sampling was carried out at high water and low water, six times a year, by towing an Agassiz trawl, with the Forth River Purification Boards' research vessel, the 'Forth Ranger'. Two residents, Crangon crangon and Pandalus montagui, and a migrant species, Crangon ailmanni, were identified as the main three species of shrimps in the estuary and Firth of Forth. C. crangon was found throughout the estuary while P. montagui was confined to the lower reaches of the estuary. C. alimanni appeared in the estuary in October and left by June. In the Firth of Forth, P. montagui and C. alimanni were the dominant species. The breeding cycle commenced in October, and berried females were found by December/January for all species. Berried females of P. montagui, and both male and female C. alimanni, migrated from the estuary to deeper areas, never to return. C. crangon females with eggs ready to hatch, spent females and larvae all occurred in the estuary. The larvae were present in the estuary from April to October. Larvae of the other two species were not found in the estuary. All species fed mainly on polychaetes, followed by bivalves and crustaceans, which indicated a benthophagous feeding habit. The choice of food depended on the local availability of prey items, and the range of the particular shrimp species within the area; shrimps fed on prey which was abundant in their area of residence rather than moving elsewhere. The Forth Estuary is well utilized by the three species with little competition between them. Although a slow growth rate was observed in C. crangon, the mean condition factor indicated that the conditions in the Forth estuary were close to those normally required for shrimps. The Forth estuary shelters three species of shrimps, with populations, varying between 1992 and 1993, of 1.6-7.7 x 107 for C. crangon, 1.6-2.5 x 107 for P. montagui and 0.7-1.0 x 107 for C. alimanni. These three species contributed to the total annual shrimp production, which ranged from 5.59-17.93 tons at low water in the ratio 40:14:1. Both resident and migratory fish species benefit from this production because shrimps play a key role in the food web, forming the major link between the lower benthic invertebrates and predatory fish.