Analysis of Sampling Methods 



In the catch- sampling program, we are 

 concerned with changes in the population 

 over its entire range of distribution, 

 hence, our estimates of its various proper- 

 ties depend on: (1) how adequately the 

 fishery sajnples the population and (2) how 

 representative our samples are of the catch 

 from which they are drawn. Although the 

 relation between the commercial catches aind 

 the population is not known at present, 

 there is little doubt that the commercial 

 catches provide the best meains of assessing 

 the composition of the population. In 

 order to devise a system of sampling which 

 would furnish a true representation of the 

 commercial catches, results obtained in 1952 

 were used as a guide. The preliminary 

 findings have been verified from additional 

 material gathered in other areas along the 

 coast in subsequent years. 



At the start of the sampling progrEun, 

 consideration was given to the selection of 

 the length measurement which was most repre- 

 sentative of the bulk of the fish. Compari- 

 sons were meide of the regressions of body 

 weight on fork length, total length, and 

 standard length ^ for 50 specimens, ranging 

 from 188 to 2 99 mm. fork length. Length 

 measurements, to the neeirest millimeter, 

 were made on a measuring board having a nose 

 block at one end and a millimeter scale 

 inlaid along the center of the bojird. The 

 fish were weighed to the nearest gram on a 

 triple-beam balance. All measurements were 

 made on fresh specimens. The 

 logarithmic standard errors of 



estimate were computed (Snedecor, 1946) and 

 coefficients of variation obtained (by 

 taking the antilogarithms). The results 

 indicated that differences among the three 

 methods of measurement were inconsequential 

 (C = 10.3, 10.2, and 10.2 percent for fork 

 length, total length, and standard length, 

 respectively). From a practical standpoint 

 total length is not the most desirable 

 measurement because the tips of the caudal 

 fin often are broken or frayed on specimens 

 Cciught by purse seines. Standard- length 

 measurements not only are more difficult to 

 perform under adverse conditions aboard the 

 vessels, but also are time consuming. Fork- 

 length measurements, on the other hand, are 

 readily obtained, and rarely are the medijin 

 rays of the caudal fin damaged. Further- 

 more, the black pigment which marks the 

 tips of the median Cciudal fin rays afford a 

 precise endpoint for measurement. Accord- 

 ingly, fork length was chosen as the measure 

 of fish length. 



Results of the first year's sampling 

 had shown that there is a distinct tendency 

 for fish of the same size eind age to school 

 together. Accordingly, the school was 

 chosen as the basic unit of sampling. Fur- 

 thermore, it was found that schools of 

 similar size and age tend to occur together 

 within an immediate area. In table 4, for 

 example, are given the length frequency 

 distributions of individual samples taken 

 from seven separate boat catches made in 

 different locations within Delaware Bay on 

 the same day. The maximum distance between 



5_/ Fork length was measured 

 from the tip of the snout 

 (with the jaws closed) to 

 the median rays in the 

 fork of the caudal fin. 



Total length is defined 

 as the straight-line dis- 

 tance from the tip of the 

 snout to the tip of the 

 caudal fin with the dorsal 

 and ventral rays pressed 

 together. 



Standard length is the 

 length from the tip of the 

 snout to the anterior 

 swelling of the hypural 

 plate. 



Table '♦.--Length frequency distributions of samples of Atlantic 

 menhaden taken from seven different locations within 

 Delaware Bay (September 9, 1952) 



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