Perspectives on Field Studies and Related Biological Models of Bivalve Growth and Carrying Capacity

Abstract

Marine bivalves are cultured throughout the world, and in many places such as Atlantic Canada and New Zealand, culture of mussels and other groups has grown exponentially in the last decade (Hickman 1989; Mallet 1989). Acceptable culture sites are limited due to habitat suitability, road access, and competing recreational or commercial use such as wild fisheries. As available culture space becomes filled up with stock, there may be a depression of individual bivalve growth rate and an increase in mortality caused by several factors associated with overcrowding. Suspension feeders have a remarkable capacity to filter the water column such that they are food limited at high culture density (Navarro et al. 1991). There are several indications that production maxima have been reached or exceeded as culture is continually expanded to the detriment of food supply (Mallet 1989; Hickman et al. 1991; Pérez Comacho et al. 1991). This phenomenon is also documented for natural populations of both infaunal and epifaunal bivalves including mussel beds (Peterson and Black 1987; Fréchette and Grant 1991; Smaal 1991; Bayne and Hawkins 1992). Moreover, culture research has demonstrated major site differences in growth rate (Mallet et al. 1986) confirming that environmental conditions can regulate shellfish production. Overcrowded culture conditions also lead to increased incidence of shellfish diseases (Dijkema and van Stralen 1989). Finally, high culture biomass may produce a negative feedback to the local environment through organic loading and anaerobic conditions beneath culture leases (Dahlbäck and Gunnarsson 1981), potentially leading to degradation of culture environments.