Fish Bulletin No. 53. Measures of Abundance of the Sardine, Sardinops caerulea, in California Waters
- Author(s): Clark, Frances N
- et al.
The proper administration of any fishery must be based on a knowledge of the abundance of the species on which that fishery relies and the variations in that abundance must be measured from year to year. Seldom can the absolute abundance of a population be measured, and for a marine species practically never. The fisheries researcher has, therefore, only one recourse; he must rely on relative measures of abundance. He can not say that a given species consisted of so many individuals in year A and a certain number in year B. He can only measure the abundance of year B in terms of year A and say that the population of year B was a certain percentage more or less than year A.
For measuring this ratio, fisheries workers have set up certain yardsticks and as a rule use that yardstick which is most applicable to the fishery under study. The most commonly used of these measures of changing relative abundance of a population are the return to the fisherman per unit of effort expended, the variations in the length of time a given year-class can be detected in the fishery, the variations in the proportions of largest fish in relation to all sizes, and the development of new fishing grounds when older, first exploited grounds fail to meet the demand for fish. In more recent years, the results of marking experiments indicate that such studies may constitute a trust-worthy measure of changing population abundance.
Frequently, no single measure of changing abundance will tell the entire tale and the use of several measures more accurately reveals the true condition of a fish population. For the sardine in California waters certain measures have been applicable for a longer time interval than have others. The last to be indicative has been the return per unit of effort. From its inception the industry expanded rapidly and no constant unit of fishing effort was expended. Each season a larger unit was applied and no unit was comparable with the former. Only in the past five or six seasons has the industry become sufficiently stabilized to make comparable from year to year the return per unit of effort. This paper presents such a return for the Monterey and San Pedro sardine fisheries from 1932–33 to 1937–38.
The unit used is based on the lunar month catches of selected boats expressed in percentages of each boat's catch for the corresponding lunar month of the previous season. The geometric mean of these percentages measures the variations in the return to the fishermen by means of the effort expended. In each locality the fishing season is made up of two more or less distinct divisions, the fall and the winter fisheries. The distinction is based on the sizes of fish caught, the fall fishery depending on smaller fish and the winter fishery on larger sizes. These fisheries have been treated separately in this study and the trends of the return per unit of effort are shown. The returns for 1932–33 are considered as 100 per cent and the succeeding seasons are expressed in percentages of 1932–33. In general the trends of all four comparisons are very similar and suggest no consistent change until 1934–35. In the three succeeding seasons, each year's fishing yielded less return than did the previous and in 1937–38 the fishermen were catching, with the same expenditure of effort, less than half as much as they had six seasons before.
A decline of more than 50 per cent in the return per unit of effort indicates a rather serious decrease in the abundance of the sardine population but an additional yardstick measuring changes over a longer time interval is desirable. This is to be found in the length of time that a given year-class now remains in the fishery as compared to former years. It has not been possible to trace the history of every year-class, but certain superabundant groups are outstanding due to exceptional survival of the spawn in some seasons. The history of several of these groups has been traced. A previous publication pointed out that formerly superabundant groups could be followed through the fishery for as much as six to ten years, but by 1932–33 this was no longer true. Such a group, which entered the fishery in 1929–30, remained in the winter fishery for three seasons only. Since that time no superabundant group has maintained a definite dominance past its second year in the fishery. This would indicate that the serious decline in abundance of the sardine population began about 1930 and has continued for the succeeding eight years. At present the population has reached such a low level and is subjected to such an intense fishery that the life span of a year-class after reaching adulthood may be not more than four or five years. This is in contrast to a former adult life span of ten years or more.
As the length of life of each year-class decreases, fewer fish remain to grow to larger sizes and the average length of the fish in the fishery should show a corresponding decline. For the past decade the California sardine fishery has taken approximately half its tonnage during the fall months when only the younger, smaller adults are present in the fishery. This places an excessive strain on the smaller sizes and allows fewer and fewer fish to grow to larger sizes. As a result, since 1932–33 there has been an almost constant decrease in the average size of the fish in the fishery.
Of the measures of relative abundance as yet applied to the sardine, there remains to be discussed the expansion of the fishing grounds. During the earlier years of the California sardine fishery, the fishing grounds were constantly extended. This was possible because larger boats with a greater cruising radius were built each year. For the past ten years, although more and larger boats were still being built, the fishing grounds off the California coast have experienced little expansion. Nearly all areas where sardines are to be found from San Francisco south to San Diego were tapped by 1930. The only change in the later seasons has been heavier fishing in the more distant areas as more and more boats capable of making such extensive trips have entered the fishery and as the nearer grounds have failed to meet the demand for fish.
As we have come to understand more clearly the life-history of the sardine, we realize that an expansion of the fishing grounds does not exploit new sardine populations. Various studies of the biology of the sardine have indicated that the sardine is a migratory species and the results of a tagging program now under way. Since the sardines are moving freely from one fishing ground to another, a given locality after being fished out may shortly be repopulated by a new group of fish. Conversely fishing in one locality takes its toll from the entire population rather than affecting only the population in the one locality. At present, the sardine fishermen are searching for fish along the California coast from Point Arena south to the Mexican boundary, except in the region around Point Buchon which is still too remote from markets to be profitably fished.
An extensive tagging program now being carried on promises to furnish, in the future, an additional measure of the sardine population abundance. The number of tags recovered per ton of fish handled should indicate the percentage of the population caught yearly by the fishermen. The study covers too few seasons as yet to permit a report on the results.