PARAMETERS OF MARINE INVERTEBRATE POPULATIONS 



D. A. Hancock and A. C. Simpson 



Fisheries Laboratory, Burnham-on-Crouch 



INTRODUCTION 



In Britain the marine invertebrates which are exploited commercially are 

 found among the Crustacea (lobsters, crabs, shrimps, prawns and Norway 

 lobsters), and Mollusca (oysters, mussels, cockles, scallops, clams, whelks and 

 winkles). They include animals with a wide range of habits — free-swimming 

 forms (shrimps, prawns), mobile bottom-living animals (lobsters, crabs, 

 Norway lobsters, whelks and winkles) and sedentary molluscs (oysters, 

 mussels, cockles, scallops and clams). Exploited mussels, cockles, clams and 

 Portuguese oysters are normally intertidal in distribution while the remaining 

 species are exploited below low-water mark. 



In Europe, only the sedentary molluscs; oysters, mussels, and to a lesser 

 extent cockles, are cultivated. Otherwise, marine invertebrates are fished 

 from wild, natural populations. Population studies in each kind of fishery 

 are directed towards the same end, namely that of rational exploitation and 

 achievement of the best long-term yield. So far, the general principles of 

 population dynamics established for fish (Beverton & Holt, 1957) have been 

 found to be apphcable, but there are with shellfish some special problems, 

 and certain advantages when examining exposed sedentary populations. 



SAMPLING THE POPULATION 



It is characteristic of most shellfish that their movements are very hmited, 

 varying from nil for the sedentary molluscs to a few miles at most for crabs 

 and lobsters. Even the migrations of shrimps and prawns seem unlikely to 

 prove very extensive. Any variations from one locality to another in the 

 animate or inanimate environment which determine the growth or survival 

 of individuals have only a local effect and are not spread widely through 

 subsequent mixing of individuals over a wide area. For example, the growth 

 rate and mortahty rate of cockles vary in different parts of a single extensive 

 cockle bed, and for lobsters these parameters may differ greatly within a 

 few miles, due to local environmental factors and the proximity of a fishing 

 port. Therefore, great care has to be taken in sampling the population either 



