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Fishery Bulletin 89(3). 1991 



oceanic color and thermal fronts apparent in the im- 

 agery. They also observed that shoreward intrusions 

 of oceanic water were coincident with notable albacore 

 aggregations. Laurs et al. (1984) based their conclu- 

 sions on a visual analysis of catch rates superimposed 

 on images, and did not present any statistical analyses, 

 such as relating catches to distances from frontal 

 regions. Fiedler and Bernard (1987) analyzed satellite 

 imagery and stomach contents data taken from 

 albacore and skipjack Katsuwonus pelamis and 

 demonstrated that these fish were opportunistically 

 feeding on prey items associated with frontal regions 

 off the California coast. 



Maul et al. (1984) utilized satellite imagery to com- 

 pare SST with Atlantic bluefin tuna Thunnus thynnus 

 thynnus catches reported by the Japanese longline 

 fishery that operated in the Gulf of Mexico during 1979 

 and 1980. The 1980 catch was substantially higher than 

 that of 1979, and Maul et al. (1984) attributed the in- 

 creased catch to the shift of fishing activities closer to 

 the frontal zone associated with the Loop Current edge. 

 They stated that 85% of the 1980 catch was taken 

 within 100 km of the Loop Current. In contrast to the 

 Laurs et al. (1984) study, much of the Maul et al. (1984) 

 analyses were quantitative, rather than qualitative, 

 since the distances from the locations of bluefin catches 

 to the edge of the Loop Current were analyzed. 



Herron et al. (1989) continued efforts to quantify the 

 relationship between fish catches and temperature 

 structures in the Gulf of Mexico by analyzing 20 sea- 

 surface temperature (SST) images acquired concurrent- 

 ly with trawl catches of the demersal butterfish Pepri- 

 lus burti. They calculated statistically significant 

 regressions relating butterfish trawl catches to SST 

 gradients computed from satellite imagery. 



In their study of tuna catch in the Gulf of Mexico, 

 Maul et al. (1984) concentrated on bluefin catches 

 relative to the edge of the Loop Current. In addition 

 to the Loop front, there are numerous other coastal 

 and oceanic regions of rapid temperature change that 

 are potentially important aggregators of tuna. For ex- 

 ample, Huh et al. (1978) described extensive coastal and 

 shelf thermal patterns in the northeastern Gulf of Mex- 

 ico that were present during 1976-77. Although exten- 

 sive cloud cover and regions of near-isothermal SST 

 values occur in the Gulf of Mexico during the summer 

 months (Huh et al. 1978), considerable variation in SST 

 is evident in satellite imagery collected during the fall 

 through spring months. Figure 1 is an image of Gulf 

 of Mexico SST patterns on 21 March 1988, and dem- 

 onstrates the intricate thermal patterns that can be 

 present in the northern Gulf of Mexico. 



A U.S. -based fishery for bluefin and yellowfin tuna 

 Thunnus albacares has rapidly developed in the Gulf 

 of Mexico. Although Japanese longline vessels har- 



vested considerable numbers of yellowfin tuna during 

 1963-81 (Wilson 1988), domestic landings prior to 1983 

 were relatively low and primarily the result of bycatch 

 from swordfish Xiphias gladius fishermen. By 1986, 

 however, Louisiana landings had leaped to 24 million 

 > pounds (Adams 1987). In the same year, 3.4 million 

 pounds of yellowfin tuna were landed in the west coast 

 of Florida, with the majority landed in the panhandle 

 region. These fishermen frequently rely on ocean- 

 surface temperature to judge where to set their lines. 

 The conventional wisdom is to set lines when a tem- 

 perature change of a "couple degrees" is detected. 

 At the present state of knowledge, assessment and 

 management of the tuna resource must depend on 

 catch statistics to indicate changes in overall stock size. 

 Catch-per-unit-of-effort (CPUE) can be elevated if 

 fishermen locate tuna that may have aggregated at 

 fronts or regions of rapid temperature change; alter- 

 natively, the CPUE may be depressed by the absence 

 of fronts or the fisherman's inability to locate them. 

 In either case, the data used for assessment and 

 management decisions could be biased to an unknown 

 extent by the possible concentrating effect of frontal 

 boundaries. This research was intended to explore 

 possible relationships between the Gulf of Mexico SST 

 structure observable in satellite imagery and the 

 yellowfin tuna catch and effort, and to determine 

 whether regions of temperature change yield increased 

 fishing success. 



Methods 



There were three primary components to the research: 

 (1) Acquisition, validation and summarization of the 

 seasonal and spatial patterns in the longline fishery 

 catch and effort data; (2) development of a satellite 

 image database and description of the seasonal and 

 spatial patterns in SST; and (3) investigation of any 

 associations between the two data sets. 



Data on longline sets were acquired from longline 

 logbook records compiled by the National Marine Fish- 

 eries Service (NMFS). Data were from domestic fishery 

 longline sets in the Gulf of Mexico and western Atlan- 

 tic spanning September 1986-December 1987. These 

 records included date, location of set, length of longline 

 in miles, number of hooks fished, and number of fish 

 caught. Catch records without valid latitude and longi- 

 tude coordinates were deleted, as were records outside 

 the Gulf of Mexico (east of 80°30'W longitude and 

 south of 6°N latitude). Data such as time of day and 

 duration of set, bait used, sizes of fish caught, or 

 whether the reported geographic coordinates repre- 

 sented the location of the beginning or end of the set, 

 were not available for our analyses. The total number 



