Fish Bulletin No. 47. Interseasonal and Intraseasonal Changes in Size of the California Sardine (Sardinops caerulea)
- Author(s): Clark, Frances N
- et al.
No true understanding of the California sardine fishery or of the sardine population can be reached until the size fluctuations which occur within each fishing season are clearly set forth. These rhythmic size changes, repeated with constant regularity season after season, must be analyzed and interpreted before any measure of other changes in the sardine fishery can be attempted. They result from successive movements of the various size-classes, composing the sardine population, into and out of each fishing area. In addition to these size changes within each sardine fishing season, there also occur significant size fluctuations from season to season. These result from an entirely different cause, the relative success or failure of each season's spawn. A measure of the amount of variation in the success of each season's spawning is as important as is the understanding of the size variations within each season. The size changes within each season are of greater magnitude, however, than are the size changes from season to season. Consequently the measure of the success of each spawning season must be based on an analysis which eliminates the effect of size variations within each season. The simplest way to do this is to divide each sardine season into units, each of which represents a time interval characterized by certain sized sardines. With the exception of the bait fishery and a limited canning of young fish at San Diego, the California sardine industry depends on adolescent and adult fish. These are not present in California fishing areas throughout the summer months in sufficient numbers to maintain the industry. As a result the sardine fishing season is confined to the fall, winter and early spring. In the two major fishing areas, Monterey and San Pedro, the fall fishery relies on smaller fish than does the winter fishery. Because of this consistent size difference, all recent conclusions drawn from the sardine studies have been based on a fall and a winter fishery and the two fisheries have been analyzed as distinct units. The data, so treated in this paper, demonstrate that, with few exceptions, each season the fall fish at Monterey are somewhat larger than are the fall fish at San Pedro, and the San Francisco fall fish are correspondingly larger than the Monterey fish. (There has never been a fall fishery at San Diego.) Certain superabundant year-classes have been discernible in the fishery and these groups have been traced from season to season. They have been followed through the fall fishery for three or four successive seasons until they grew to sizes larger than those taken in the fall months. These findings are interpreted as indicating that any given year-class will be taken in the fall fishery for three or four seasons only. On the other hand, the sardines from which the winter fishery draws evince quite different characteristics. They represent a much larger size range. Fish as small as those taken in the fall also are present in the winter, but in addition there occurs a high percentage of larger, older fish. In contrast to fall conditions, the winter fish at all fishing localities from San Francisco to San Diego are similar in size. The sizes have varied from season to season but for each season similar size-classes have been found in all the winter fisheries. The same superabundant year-classes discernible in the fall fishery have also been traced through the winter fishery, although the evidence is not as clear cut. But, because the winter fish comprise older as well as younger fish, these exceptionally abundant year-classes can be followed for a greater number of seasons. Some of them have been apparent for ten years or longer and have exerted a dominating influence for six or seven years. Since during the winter months all adult size-classes appear in the sardine catch, this fishery furnishes the best basis for a study of significant changes in the population and makes possible some measure of man's inroad on the supply. The length frequency polygons and deviation curves as herein presented indicate that the dominating factors influencing the changes in population abundance were, in the early years of the industry, the presence of these superabundant year-classes. These persisted in the fishery in spite of the moderately heavy toll taken by man. In the later years new superabundant groups have entered the fishery, but due to the greatly increased fishing intensity they can no longer be traced for the normal eight to ten seasons. These later groups have begun to lose their dominance in two or three years and after the third year have been below normal in abundance. This indicates that the present intense fishery is capable of reducing the number of fish of a year-class to a very low level within three or four years after the group appears in the fishery. Under normal conditions a year-class should retain sufficient numbers to make it important to the fishery for eight or ten years. This constitutes a sign of overfishing which in the near future may prove very serious.