Estimating a Population of Shrimp by the Use of Catch Per Effort and Tagging Data
Abstract:A study of some methods used to obtain information on the dynamics of the Tortugas pink shrimp (Penaeus duorarum) fishery in south Florida is given. Tagging experiments were used together with catch and effort data to find population size, fishing mortality, immigration, and a combined figure for emigration and natural mortality. Since the data employed were inadequate in some respects for an accurate population estimate, the emphasis of the study was placed on the examination of methods used rather than the actual answers obtained. The fishing-success (DeLury) method was used concurrently on both tagged and untagged shrimp populations. This method involves the study of catchability curves which represent the relationship of the catch per effort to the accumulated catch and permits estimates of population size.
The Peterson method and the fishing-success method were used to determine population size simultaneously, thereby providing a partial check on the results.
Catches were compiled daily from records of dealers, and from these data the total catch of the fishery was estimated. In addition, landings of a sample of vessels of the fleet were recorded through interviews, and from these data the daily fishing effort was estimated. Because the number of interviews collected was low on some days, an effort was made to strengthen these data by relating nights fished to average daily wind velocity. This permitted all interviews obtained for any wind velocity to be totaled to calculate fishing effort for any day. During the period from January 12 to March 15, 1958, the catch ability curve was convex and suggested that emigration and natural mortality were not balanced by immigration or that the gear efficiency had changed during the period. It was not possible to make a population estimate by the fishing-success method. On the other hand, the period from November 12 to December 31, 1958, showed a straight line relationship representing catchability, and a population estimate of 300 × 104 pounds of shrimp was made. This estimate involved immigration, emigration and natural mortality.
The tagged population into which there was no immigration was studied to find a DeLury estimate of the size of the population of tagged shrimp present on the grounds at the outset of the tagging experiment. About 25 per cent of the tags present on the grounds were recovered.
By using the catch ability rate from the tagged population, a true catchability rate was determined which is not influenced by emigration or immigration. The untagged population estimate was adjusted (modified DeLury estimate), and this figure of 374 × 104 pounds was compared with a Petersen estimate of 392 × 104 pounds by the use of recoveries obtained near the beginning of the experiment. The good agreement between these two population estimates (within 4 per cent) suggests that the rates of immigration and emigration plus natural mortality are approximately correct. The tentative conclusion reached is that during November-December, 1958, the size of the population on the grounds was considerably below the figures for both immigration and emigration plus natural mortality.
On the basis of this study, recommendations were made for experiments to provide reliable estimates of the parameters of the fishery. These recommendations include larger and more complete tagging experiments and improved estimates of fishing effort from interviews.
Document Type: Research Article
Publication date: 1962-01-01
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