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Swordfish, Tunas and Temperature
by:: Edward R. Gaw
PRINTED:: Fishing Boat World June 1998

Contrary to popular belief, swordfish tunas and sharks are not cold-blooded fishes. Through a combination of counter current heat exchangers, preferential blood flow distribution and brain heaters (rete mirabili), these fish work to maintain a preferred internal temperature range. An understanding of this specific temperature preference and the influence of temperature on the local availability of prey species can dramatically improve hook and line catch rates.

The complex influence of the effect of water temperature on the behavior of target species, swordfish and tunas, and the predictable location of bait can be simply stated. Colder or cooler water is denser or heavier than relatively warmer water. Close association of two (2) or more bodies of water, distinguished by different temperatures is defined as a temperature front. The evolutionary stages of any temperature front include its birth, life and disappearance. Frontal strength and its relative longevity can be measured by the size of the temperature gradient across the water masses involved. It is extremely important to remember that water temperature figures strongly in water density. Given that colder water is heavier and warmer water lighter, interpretation of temperature fronts can be seen as the developing interaction between density surfaces. The size of the initial temperature gradient will determine the strength and relative thickness of the density surfaces present.

Fisheries interpretation of the presence of a temperature break or existing density surfaces in an oceanic context, allows for the progressive accumulation of particles and prey items on or around these density differences. As wind borne or current propelled particles settle through the water column, the existence of a strong density gradient will collect particles and activity over time. Successive development will attract and foster the development of a food web around any density feature. Progressively larger organisms drawn to this natural accumulation of forage items, culminate in elevated bait activity and target fish availability. The effect of density on hook and line success for swordfish and tunas is direct and documented.

Events producing local increases in the presence of swordfish and tunas, take time to develop. The collision or interaction of different temperature water masses produces a strong surface signature. These features are identifiable from remote sensing observations. Satellites, specifically sea surface temperature (SST) measurements document the creation and existence of a potentially productive piece of water. However, satellite derived surface temperature information is extremely sensitive to atmospheric moisture and this data only integrates the water temperature through the first several millimeters of the observed water�s surface. For this reason, SST satellites are valuable tools in the early identification and movement of potentially �good� water but maintain limited utility in the hook and line capture of swordfish and tunas.

The influence of daily solar heating and nighttime cooling dissipates any surface temperature signature over time. The sun�s influence on an ocean�s surface layer smoothes the presence of any front and ultimately renders it virtually invisible from any remote platform. At the same time the sun is masking the presence of any temperature or density feature, this feature is growing more productive. Consequently, at its most productive fish gathering strength, the aged temperature front is relatively invisible to SST satellites. Consistent location and knowledge of these older productive density fronts separates consistently productive captains from occasionally lucky ones.

The presence of commercial quantities of swordfish and tunas requires the integration of both remote satellite information and local measurements. Trip planning should include satellite remote time series indicating the creation of density/temperature fronts and more importantly, their relative movement. Despite the effect of time on frontal visibility, exercised tracking of frontal features will serve to assist in the reliable prediction of productive feeding grounds. Interaction of these mobile, transient features with bathymetric structure; continental shelf/slope, seamounts, banks, will compound a feature�s ability to draw and hold fish.

Despite all the satellite information and vessel charting and tracking efforts, the decision to fish and throw gear in the water is a local one. Most well equipped vessels carry an advanced HI-LINER ATA Temperature Gauge. This instrument is the only piece of commercially affordable electronics available that accurately measures SSTs to the hundredth (0.01) of a degree. This meter has proved invaluable in the pursuit of developed productive temperature/density features in tropical and subtropical situations, subject to persistent daily solar heating. Through the learned use of this powerful fishing instrument, it is possible to identify productive �good� water. The display of subtle changes in temperature and the revealed presence of distinctive temperature patterns can improve the performance of any vessel, regardless of target species. This surface temperature device has been the secret of the most productive captains for years. The HI-LINER ATA PLUS Surface Temperature Gauge has proved a significant advance in fishing technology and gear improvement.

There remains no substitute for the disciplined charting of satellite water features over time. Despite a SST satellite�s best efforts to reveal a productive water mass, it is a feature�s course and persistence through time that determines its ability to collect target animals and create a favorable fishing environment. The successful application of hook and line fishing gear requires a spread of hooks in areas rich in local bait concentrations. One strong mechanism for increasing local bait availability is the presence of a pronounced thermal front and accompanying density surfaces. Successive accumulation of particles, phytoplankyon, zooplankton and progressively larger baitfishes culminate in higher local concentrations of feeding target animals, swordfish and tunas. The presence of a correctly baited hook and FLEX-STICK light-stick in this environment significantly increases the probability of capture and success.

Successful long-lining comes from a basic understanding of the interaction of many different variables. The importance of these factors vary but a vessels ability to consistently locate �good� water strongly influence its catch history and profitability. Because the prey species of swordfish and tuna are creatures of the edge, it is possible to improve the placement of pelagic long-line gear within a fishing area. Vertical limitations of monofilament long-line gear demands constant refinement of gear construction and materials. Patterning fish behavior by scouting a particular location with different gear parameters allows refinement over the course of a fishing trip. However, it remains crucial to remember that a lack of initial success in the adjustment of any gear component does not equal failure. It is strongly recommended to incorporate some degree of gear variability in each set made. A reasoned approach to the aging and development of temperature fronts will ultimately improve the fishing power of any correctly baited hook. A more detailed explanation of swordfish, tunas and temperature and its influence on pelagic monofilament long-line gear use can be found in; LONG- LINE FISHING FOR BIG FISH, by Walter Flanagan and Edward R. Gaw.
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