42 D. A. HANCOCK AND A. C. SIMPSON 



FISHING MORTALITY 



If a reliable estimate of the stock has been obtained by direct survey or by 

 marking experiments, the fishing mortahty can be calculated from it, using 

 figures for landings from the fishery. This involves the collection of accurate 

 catch statistics, which unfortunately are not always available for shellfish, 

 due to the difficulty of collection. Fishing mortality may also be assessed 

 from marking experiments, which have so far been used for crabs, lobsters 

 (Thomas, 1955^) and whelks. A knowledge of factors which normally 

 complicate the interpretation of data from marking experiments is necessary. 

 Shedding of marks may be of prime importance in Crustacea, but migrations 

 are less of a problem with shellfish than with fish. Reinforcement of the 

 unmarked part of the population may, however, occur from areas which are 

 unsuitable for exploitation. This should be considered when planning the 

 distribution of marked individuals in tagging experiments. With sedentary 

 populations a separate estimate of natural mortality can be made, and related 

 to the total density to give the fishing mortality. 



The problems of sampling have been mentioned, and these are important 

 when calculating catch per unit of effort from commercial landings. Selective 

 fishing mortahty of certain size groups occurs in the action of dredges, 

 trawls or traps, and also in the sieves used for riddling cockles (Fig. 3). In 

 some cases a minimum landing size is enforced by law. This may involve 

 restrictions on the size of the mesh of the gear used (shrimps, prawns), or 

 on the size of the shellfish (all others) which may be taken. 



Certain fisheries take place only in restricted seasons, which may vary in 

 extent in different areas along the coast. This may be the result of national 

 legislation (e.g. close season for native oysters), locally administered byelaws 

 (e.g. close season for mussels in South Wales Sea Fisheries District) or merely 

 the voluntary pursuance of an alternative fishery. An example of this is the 

 whelk fishery of Grimsby, which is confmed to the winter months, while 

 lobster fishing is more lucrative during the summer. In all fisheries with 

 baited pots the effective effort is greatly reduced by bad weather (Fig. 2). 



A direct result of the limited amount of mixing which occurs in shellfish 

 populations is the selective fishing of the fastest growing individuals, par- 

 ticularly when there is a minimum size regulation. For example, each summer 

 on heavily exploited cockle beds the fishermen await the growing to size of 

 cockles. The areas of fastest growth are fished first, followed by the slower 

 growing areas, but there will be repeated visits to the areas of fastest growth 

 as cockles reach market size. Since a higher proportion of cockles reach 

 market size in the faster growing areas, the fishing mortahty is consequently 

 greater there. All over the beds, the fastest growing cockles are fished off 



