of the Fishery Board for Scotland. 333 



encountered in determining the true limits of a series. In many cases a 

 group may be insufficiently represented by the absence of a due proportion 

 of either the larger or the smaller fishes. Tlie latter may be owing to 

 imperfection of the apparatus of capture, as already explained, the smaller 

 fishes, present on the ground, escaping from the net ; or it may be due to 

 difference of habitat, as, for example, in the case of the young whiting and 

 haddock, which are pelagic, and the young plaice, found only in the shallow 

 water while the larger individuals are in deeper water. In other cases the 

 imperfect representation may arise from the migration of part of a series, 

 as with the larger haddocks and whitings in autumn and winter in 

 Aberdeen Bay, or to their irregular roving movements as with the 

 cod. In the latter case it may happen that one haul in a particular 

 locality furnishes one part of a series and a second haul the other part. 

 Moreover, it often happens that while the division between one series 

 and another is distinctly indicated in one haul, it may be obscure in 

 another haul owing to the capture of an undue proportion of the larger 

 fishes of one series and the smaller fishes of the next older series. This, 

 curiously, occurred most commonly in the deep water off Aberdeen, the 

 hauls in the deep water far from land (ofE the Shetlands), being, as a rule, 

 the most satisfactory. Another example of the coalescence of groups is 

 found with the plaice when the fishing is carried on within a limited range 

 of depth (see p. 347). 



Besides all these causes, which are temporary or fortuitous, diiJiculties 

 arise from the natural overlapping or coalescence of groups due to dif- 

 ferences in the rate of growth among individual members of one and the 

 same series, and in the average growth of successive series. The utility 

 of the method depends altogether upon the circumstance that repro- 

 duction is restricted to a portion of the year, so that the brood of a 

 particular species are all born within a limited space of time — usually 

 a few months in spring — which is separated from the preceding repro- 

 ductive period by a considerable interval. If the average rate of growth 

 of successive generations were .similar, then, notwithstanding the variation 

 in growth of the members of one and the same generation, the difllculty 

 of separating the various series from one another would not be great. But 

 the average growth in length is slower in successive generations, and this 

 becomes especially marked after maturity is reached, so that the larger 

 individuals of a younger series are longer than the smaller individuals of 

 the next older series, and this coalescence or overlapping of the groups 

 increases with age. As a rule, however, the young fishes under one year 

 of age are all, or almost all, smaller than the fishes of the previous gene- 

 ration, and the interval between the two series is the greater the earlier 

 the period after the spawning season. The variation in growth among the 

 individuals of one series depends to a large extent upon the duration of 

 the spawning season, but it is also influenced by the relation to tempera- 

 ture, e.r/., plaice and gurnard. 



Another circumstance that tends in certain cases to obscure the 

 division between the series is the different rate of gi-owth of the sexes. 

 Among round fishes this is not so obvious, because the sexes are 

 apparently sub-equal in length at all stages. Thus, of 957 cod, of all 

 sizes, examined, the length of the female to the male at 100 was 9.5; 

 of 1375 haddocks, the proportional length of the female was 98 ; and of 

 1318 whitings, the proportional length of the female was 104 to the 

 male at 1 00. The difference is thus not very great ; but at the .same 

 time, looking to the difticulty of separating the groups from overlapping, 

 better results woidd be obtained by treating the sexes separately, and 

 this might be done at the spawning season without many fishes requiring 



