HCNSO Student Theses and Dissertations

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Defense Date


Document Type

Thesis - NSU Access Only

Degree Name

M.S. Marine Biology


Oceanographic Center

First Advisor

David W. Kerstetter

Second Advisor

Alexander V. Soloviev

Third Advisor

John F. Walter III


The pelagic longline is a common gear type used worldwide primarily for the commercial harvest of swordfish (Xiphias gladius) and various species of tuna (Thunnus spp.). Different species are targeted by deploying the longline gear at different depths. This technique is effective because the probability that a fish will consume a bait is proportional to the amount of time that particular bait spends in the fish’s feeding habitat. The depth of the gear can be altered by changing the number of hooks per basket, with the gear being shallower with fewer hooks. Shallow longline sets are typically used when targeting swordfish and yellowfin tuna (Thunnus albacares), and may utilize four to six hooks per basket. The average maximum shallow-set gear depth is about 120 meters, but there is often variation in the depth range of the set because there are other forces acting on the longline gear than just gravity and buoyancy. Wind speed and direction, vertical current shear, fish captured by the gear along the line, and occasional interactions with other ships are all factors that can have a major impact on the depth of hooks during a set. To determine which factors have the largest impact on the depth range of pelagic longline gear, small temperature-depth recorders (TDRs) were deployed over two fishing seasons for a total of 70 longline sets. These sets were deployed in several different geographical areas within the Western North Atlantic. Depth profiles for several hooks per set were obtained and compared with the predicted depths. Observed hook depths were significantly shallower than the predicted depths in over 90% of cases. On average, the observed hook depth reached only 41% of its predicted value. These differences are compared between different regions and oceanographic conditions to assess if location or sea-surface state has a significant impact on the depth of longline gear. Wind speed was found to have a significant impact on the depth of longline gear, as well as wave height. The speed at which the gear moves through the water, how the gear was set with respect to current, and the geographical location of the gear also had significant effects on the mean settled depth of hooks within a set. The differences in gear depths between geographic locations were most likely caused by a combination of local environmental variations and setting technique.


Data collection funded by CRP #NA03NMF454O42O.

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