This article originally appeared in The Shark Tagger-1997 Annual Summary
In the springs of 1989 and 1991 members of the Apex Predators Program (APP) onboard the NOAA Ship R/V Delaware II conducted two longline surveys of sharks along the Atlantic coast using Yankee Style longline gear. The 1991 survey was conducted between Tampa Bay, FL and southern New England (DE II 89-03). The 1991 survey was con ducted between Miami, FL and southern New England during the same season and used the same fishing methods (DE II 91-06).
A third survey, conducted in 1996, attempted to repeat stations from the first two surveys in the region from Key West, FL to Delaware Bay, DE during the same time season (Pelican 96-01). Gear and soak times were altered in 1996 to more accurately represent those of the commercial large coastal shank fishery; biological studies of sharks were essentially the same. Florida Style Monofilament gear (Mono) was used, replacing the Yankee gear, spiny dogfish replaced mackerel as bait, and soak times were increased from one to three hours. In addition, the survey was conducted from the beach to 40 fathoms (fm), eliminating all previously fished stations outside this depth range. To enable a comparison of the results of the 1989 and 1991 surveys with the results of the 1996 and future surveys, a gear comparison cruise was conducted during Jan-Feb 1997.
The objective of the 1997 cruise was to evaluate possible conversion factors for transforming the data from the 1989 and 1991 surveys to allow direct comparison to the 1996 data. Data were also collected to compare the difference in catch between SEFSC Shark Surveys (100 hooks/one-hour soak) and those of the NEFSC (300 hooks/three-hour soak).
Three types of stations were alternately fished: 100 hooks of Mono gear fished for 1 hour using spiny dogfish as bait (M1); 100 hooks of Yankee gear fished for 1 hour using mackerel as bail (Y1); and 300 hooks of Mono gear fished for 3 hours using spiny dogfish (M3). Randomly chosen stations were located between Cape Fear, NC and Ponce Inlet, FL between 5 and 30 fm. A random gear order (M1, Y1, M3) was chosen prior to departure and fished alternately, one type at each station.
Ninety-nine fish (93 sharks) were caught on 42 longline sets, including 8 species of sharks and 3 species of teleosts or rays. Seventy-three fish (74%) were tagged and released, 10 (10%) were brought aboard, 15 (15%) were released untagged, and one was lost. Shark catch per unit effort (CPUE) was 52 per 10,000 hook hours (1.4/100 hooks). Sharks represented 94% of the catch, of which Atlantic sharpnose sharks were the most common, followed by tiger sharks. Eighteen sets were conducted during the day (start set before sunset) and 20 at night (start set before sunrise). Set depth ranged from 6.0 to 24.4 fm, with the majority of sets in 15 to 20 fm range (Figure 1).
Although an insufficient number of sharks were caught to allow statistical comparison of the data, interesting trends can be observed (Figure 2). Overall, it appears that the Y1 gear was more efficient at catching sharks than the M1 or M3 gear. However several factors, including temperature and time of day, need to be addressed.
It is apparent that the number of sharks caught per set increased with temperature for all gear types with the most dramatic increase occurring at approximately 18.5 °C (Figure 3). Since temperature increased linearly with depth (Figure 2), the catch/depth relationship mimicked that of the catch/temperature relationship. On average, the Y1 gear was fished at a greater temperature than the M3 and M1 gears. In particular, there is a 2 °C difference in mean temperature between Y1 and M3 stations; this discrepancy could have contributed to the catch differences.
It is also interesting to note that while the Y1 gear caught more sharks, the majority were small coastals. When the catch is split by species groups comprising the shark management units, the difference in catch of large coastals by gear types becomes less dramatic, with the only sandbars being caught on the M3 gear.
Another factor to consider in relating catch to gear is day versus night sets. On this cruise, the catches were greater during the day than at night. Only 4 of 14 M1 sets were conducted during the day, whereas, the Y1 sets were more equally divided (7 day/6 night) and the M3 gear had more day (7) than night (4) sets.
Though the cruise was set up to be random in terms of all possible influences, it is apparent that some bias might have occurred which could have influenced the results. In the future, as this type of comparison is continued and refined, the stations will be closely monitored to equalize all these factors including locations, times, and temperature. Additionally, we may change the time of year for the survey in order to increase the possibility of obtaining sharks.