NATURE OF VARIABILITY IN TRAWL CATCHES 



By CLYDE C. TAYLOR, Fishery Research Biologist 



A fundamental problem in deriving the char- 

 acteristics of a population from a series of samples 

 is the determination of the magnitude and sources 

 of variability in successive samples. Such vari- 

 ability may arise (1) from the manner in which 

 the population is distributed in space and (2) 

 from variations inherent in the method selected 

 for sampling. This paper considers the variability 

 associated with catches of fish by the otter trawl 

 with respect to such sources. Aspects of this 

 problem have been considered by Thompson 

 (1928), Hickling (1933), and Barnes and Bagenal 

 (1951). Parallel problems for plankton have 

 been treated by Winsor and YYalford (1936), 

 Winsor and Clarke (1940), Silliman (1946), and 

 Barnes (1919a, 1949b, 1951). 



Winsor and Walford considered that "the 

 variation actually observed in plankton sampling 

 is the result of (a) variations in the distri- 

 bution of the organisms and (b) variations in 

 the mode of sampling. Under (b) are included 

 such factors as variations in the speed of hauling, 

 deviations of the haul from the vertical, com- 

 pleteness of washing the net, etc." It is obvious 

 that similar factors under (a) anil (b) would affect 

 the variability of trawl hauls. 



Since the chi-square distribution of paired 

 plankton hauls did not satisfy that expected for a 

 Poisson distribution of the organisms, Winsor and 

 Walford examined the variability arising from 

 factors under (b) and concluded that "a limit to 

 the precision of estimate is set by the relative 

 variability of the volume sampled." 



Silliman (1946) studied the variability in 

 plankton-tow-net catches of Pacific pilchard eggs. 

 He concluded that "most of the variability of egg 

 counts is due to real differences in egg concen- 

 tration." 



Barnes (1949a) studied the variability in paired 

 plankton hauls. Following a treatment of the 

 data parallel to that of Winsor and Clarke, he 



concluded that his results "are not at variance 

 with the suggestion that the animals are randomly 

 distributed, and that the variations observed are 

 largely dependent upon variation in the volume of 

 water filtered by the net." 



Barnes (1949b) proceeded to examine the 

 variability arising from variations in the volume 

 of water sampled, by comparing the catches of 

 plankton organisms taken by pump samples in 

 which the volumes of water actually sampled 

 could be determined with considerable accuracy. 

 Barnes found that the variability associated with 

 pump samples was no less than that with net 

 samples and that the distribution of chi-square 

 was similar to that found by Winsor and Walford 

 (1936). Barnes' contribution indicates clearly the 

 necessity for reexamining the assumptions con- 

 cerning the distribution of the organisms in 

 space. 1 



SAMPLING PROCEDURE 



A census of fish populations on Georges Bank 

 (Area XXII. subareas (i, 11, -I, M, X, and O, 

 tig. I), using the research vessel Albatross III, 

 was completed for the summers of 1948, 1949, 

 and 1950. A stratified random selection of sta- 

 tions was designed to sample uniformly the depth 

 zones of the bank from 10 to 200 fathoms. At 

 each station a standard half-hour tow was made 

 using a modified No. 1% Iceland trawl with belly 

 and cod-end liners of 1 '.-inch-stretch cotton mesh 

 in order to catch fish as small as possible. At 

 each trawl station, all fish were counted, repre- 

 sentative samples were measured, and tempera- 

 ture data from surface to bottom were obtained 

 by the bathythermograph. A second tow was 

 made at randomly selected stations in 1948 and 

 at every third station in 1949 and 1950. 



> Assuming a heterogeneous distribution of plankton organisms, Barnes 

 has shown in a recent study that the distribution of plankton -tow-net 

 catches conforms satisfactorily to the "contagious" distribution of Thomas 

 (1949). See Barnes (1951). 



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