abundant or scarce. Abundance is defined as 

 the absolute weight of fish accessible to the 

 fishery, as affected by availability. 



Several factors should be kept in mind when 

 using catch per hour as an index of compara- 

 tive bottomfish abundance. Because the index 

 reflects is a constantly changing quantity its 

 values are accurate only for a short time and 

 a specific locality. Adequate catch and effort 

 data that are well distributed in time and space 

 must therefore be used. To eliminate bias due 

 to insufficient data, I deleted the values for 

 those months in which effort totalled less than 

 100 hours. The population estimate used in 

 this study does not account for bottomfish 

 caught by shrimp vessels operating each month 

 of the year in the north-central Gulf. The 

 annual discard of fish at sea during shrimping 

 operations may amount to several thousand 

 tons, but an accurate measure of its magnitude 

 has never been obtained. As a consequence, 

 the calculated statistics give an incomplete 

 picture of total effort expenditure and harvest 

 of bottomfish in the northern Gulf coast 

 area and permit assessment of only those 

 portions of the resource yielding to the com- 

 mercial bottomfish fishery. The premise that 

 all trawlers are equally efficient is ques- 

 tionable since studies have shown that larger 

 trawlers with greater horsepower and speed 

 tend to catch more fish per unit time than 

 smaller vessels. These factors and limita- 

 tions undoubtedly cause error and bias, and 

 conceivably affect the accuracy of the abund- 

 ance index. Nevertheless, such an estimator 

 provides a rough measure of the broad changes 

 in the bottomfish population. 



The comparative approach is used in an 

 attempt to make a quantitative analysis of the 

 bottomfish population on northern Gulf fishing 

 grounds. Monthly and annual indices are plotted 

 to compare variations in population abundance 

 within and between fishing areas. Analysis and 



interpretations of calculated trends seek to 

 show the effect of fishing on population abun- 

 dance. Conclusions are made as to whether the 

 overall bottomfish resource has adequately 

 maintained itself during the study period, 

 and projections are made regarding future 

 levels of harvest intensity. 



West Delta .- -Comparative interpretation of 

 figure 7A is difficult because of limited 

 data for both the nearshore and offshore 

 grounds. Seasonal peaks, however, are dis- 

 tinguishable and show that large concentrations 

 (an index of 1 or more tons per hour) of 

 bottomfish were usually available nearshore 

 in June and July. It is significant that mod- 

 erate (1/2- 1 ton) to large (over 1 ton) con- 

 centrations of fish are often present offshore 

 between June and August when little or no 

 fishing activity takes place. This situation 

 apparently indicates that fishermen prefer 

 to trawl in nearshore areas . Fishing operations 

 nearshore and offshore seemed to be charac- 

 terized by abrupt and marked fluctuations in 

 catch per hour which occurred simultaneously 

 in each area. Otherwise, abundance levels were 

 generally similar on both grounds for the 

 remaining months in which values are com- 

 parable. 



East Delta . --F 1 u c tua t i o n s of bottomfish 

 abundance on the nearshore grounds east of 

 the Delta generally coincided with those shown 

 by curves derived from data of operations on 

 the offshore grounds (figure 7B). Overall 

 seasonal trends in abundance were nearly 

 the same on both grounds, with most values at 

 similar levels. In most instances where data 

 were available, bottomfish abundance on the 

 offshore grounds was moderate to great from 

 June through August, the same condition as 

 was noted for the West Delta area. 



Population data for nearshore and offshore 

 stocks showed appreciable similarity on West 



2.0- 



1.0- 



A. WEST OF DELTA 



v° 



A 1 A 



W 



AW- 



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1.0- 



• -Offshore 

 o - Nearshore 



B. EAST OF DELTA 



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W 



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1959 I960 1961 1962 1963 1959-1963 



Figure 7.--Average catch of bottomfish per hour of trawling along the 

 north-central coast of the Gulf of Mexico, 1959-63. 



15 



