PARAMETERS OF MARINE INVERTEBRATE POPULATIONS 33 



Whitstable, had only slight increases in the catches on subsequent days — due 

 possibly to reduced attractiveness of the bait, and to fishing out the area over 

 which the scent of the bait was effective. 



(8) Gear saturation by exploited and/or competing species. Whelk pots are 

 frequently filled by whelks, shore crabs, starfish and hermit crabs. In (7) and 

 (8) there is a problem similar to that described by Kennedy (195 1) for pelagic 

 fish caught in gill nets. 



When sampling for the size and sex composition of the stock biases may 

 be introduced due to the differential behaviour of sections of the stock at 

 different times of the year: 



(i) During moulting lobsters and crabs do not feed and therefore that part 

 of the stock which is moulting at any time is under-represented in the catch. 



(2) While carrying eggs the female crab is rarely caught in traps. 



(3) Traps with different-sized entrances catch and retain the largest and 

 smallest crabs with different efficiencies. 



While it is probable that most of these sources of error can be avoided or 

 measured it would appear that much more work is required on problems of 

 sampling by traps. On the other hand, the behaviour patterns giving rise to 

 these effects may be studied by the planned use of traps and the analysis of 

 systematic trap data. 



GROWTH AND AGE AT HRST MATURITY 



(l) METHODS 



The determination of growth rates among the molluscs is fairly straight- 

 forw^ard. The easiest to study are cockles (Orton, 1 926) and scallops (Mason, 

 I957)> which usually have well-defmed 'annual' rings, representing the 

 cessation of growth during each winter. It is wise to follow a single popula- 

 tion over several years, in order to ensure that the true growth pattern is not 

 confused by the presence of interference rings which sometimes occur as a 

 result of environmental changes. The ease with which the previous growth 

 history of these species can be determined from the rings may prove useful 

 for providing data at present difficult to obtain in fish, due to errors in 'back 

 calculation' interpretation of scale and ^otolith measurements for age/length 

 determinations. A great advantage in examining sedentary molluscs is that 

 the mean size and abundance of each of the pre-recruit year groups can be 

 determined. With molluscs other than cockles and scallops age determination 

 is more difficult, but growth increments can be estimated from marking 

 experiments and size frequency data. Whelks have been tagged by painting 

 the spire, and by scrubbing the periostracal layer from the area round the 

 shell lip. Subsequent new growth was determined by the presence of the 



