8.1 Sardinella (S. aurita and S. maderensis)
8.2 Horse mackerel (T. trachurus, T. trecae and C. rhonchus)
8.3 Mackerel (Scomber japonicus)
Statistics on fishing effort and corresponding catches were available for several fleets. However, the Working Group met with considerable difficulties in the utilization of these data on effort or cpue for indices of mortality or abundance of the stocks in question. The main reasons for these difficulties were:
(i) saturation effects;
(ii) changes in target species (carrying forward of fishing effort); and
(iii) changes in overall fishing patterns, especially as a
result of changes in the limits of territorial waters.
In addition customary prudence had to be exercised in
foreseeing the trends which might exist in the evolution of a fishing capacity
or catching efficiency. In a general way, this was done by taking into account
the effort or cpue for specific categories of vessels as, for example, the
Polish type B.23.
Saturation effects are common among pelagic species and sometimes lead to serious underestimation of variations in the abundance of stocks. The majority of data available on effort were expressed in terms of number of days at sea or number of days ashore. The actual amount of fish caught per day may not be constant. When fish is abundant, one or two hauls of the net may be enough to ensure ample supplies for the processing plants or for storage on board fishing vessels, on board the main factory ship or ashore. The volume caught per day then reflects handling capacity or the carrying capacity of the vessel rather than the abundance of stocks. It would therefore be desirable to collect more detail data on effort than the number of fishing days, especially in the case of purse seine vessels. Preferably these data should give information on the time devoted in search of fish concentrations as well as on the number of net hauls, for example.
Potentially most fleets are interested in several pelagic species. Thus when the abundance of one species decreases they can turn their attention to another. As a result, cpue concerning the first species decreases more rapidly than the actual decrease in abundance, while cpue for the second species will increase or decrease less quickly than its real abundance. This seems to have happened in certain cases, in particular several ocean-going fleets have recently become more interested in sardines at the southern extreme of its geographic distribution. In order to solve this problem it is recommended that countries which may have available data should follow the procedure recommended by ICSEAF and recalculate their former data, taking into account for each species only the data (catches per species and by day) relative to periods during which this species was sought after more than others, for example, when it made up 30% or more of total catch. It was admitted that this procedure could lead to cpue which underestimated changes in actual abundance. All the same, since this possible bias would run in the direction contrary to the cpue bias obtained by dividing total catch by total effort, it was thought that an examination of both series of cpue values would give the real trends of the abundance.
Statistics on cpue for sardinella from different fleets are given in Table 8, Those attributed to Sardinella aurita alone are separated from those belonging to unidentified sardinella. The only long series is that concerning Dakar sardine purse-seine fleet. Data from other fleets only cover short periods and no clear trend appears to be trustworthy in those values. Data from Dakar are mainly on young fish (< 25 cm) and only concern concentrations from one of the two nurseries of this stock. Consequently it is not possible to determine the evolution of the stock as a whole. However recruitment from the southern nursery after two periods around 1967 and 1972 now seem to be at a low level,
Table 9 shows statistics on cpue and total catches of Trachurus spp. and Caranx rhonchus. Although no distinction is made between the species, it would seem that the percentage of each has remained rather constant (T. trachurus 60-67%, T. trecae 20-30% and C. rhonchus 10%) so that each series of data could represent the trends for each species. Although there are differences between one fleet and another, it is generally agreed that most of them show an increase in their cpue. In order to facilitate the comparison of cpue trends for each fleet, the value for each year was expressed in terms of average cpue percentage for each fleet in the years 1970/73. The average of these percentages gave a general cpue index (Table 10).
The cpue statistics on this species in the divisions 34.1.3 and 34.3.1 are given in Table 10. They clearly show a decreasing tendency since about 1968 (Fig. 5). As apparent uniformity existed among the fleets a common cpue index was calculated by taking the years 1970 to 1973 as reference period as in the case of horse mackerel.
