Tropical Tunas and the ENSO Cycle
Gary SharpCenter for Climate/Ocean Resources Study
Monterey Bay, CA. USA
gsharp@montereybay.com
This thickens the western ocean tropical habitat, where tropical tunas thrive, and heats the upper levels to maximum levels. The several species of tropical tunas that inhabit these warm layers are affected in different ways, depending upon the patterns that they encounter at different stages in their development. Skipjack tuna (Katsuwonus pelamis) are tied to the highly oxygenated upper layers due to their extreme metabolic rates associated with maintaining hydrostatic equilibrium, i.e., so that they do not sink out of their habitat). This situation causes a generic physiological response in that such organisms, be they fish or other, tend to expend more of their available food energies on swimming than on growth. In doing this they tend to be relatively smaller at age, and then tend to develop reproductive capabilities earlier rather than building bigger bodies.
Another group of tropical species tends to drop down into the water column, where, typically, the ocean is cooler, hence less stressful. The limitation in this case is that there must be adequate oxygen available for them to swim, maintain hydrostatic equilibrium, and grow, or they too, will be forced into reproduction rather than somatic growth. The tropical ocean has variously been populated and adapted to by several species of tunas, with unique swimming energetics, hence oxygen requirements, and temperature tolerances, that reflect these tradeoffs. If one were to drop a line with an array of hooks into the depths of the western topical oceans, as many as six species of tunas can be caught at any one location, but they will have very different distributions amongst the hooks by depth.
Now, if we quickly travel to the eastern tropical oceans, the usual upper ocean upwelling tends to bring oxygen-poor cool water near the surface. This happens for the simple reason that the cooler source waters underlie some of the more productive tropical surface waters on earth, and due to the excess production, the night time metabolism of the plankton tends to extract most of the available oxygen. This reduces the availability of oxygen rapidly with depth, and excludes most of the species of tunas found in the oxygenated western tropical oceans. This means that the same fishing experiment within the eastern tropical ocean will yield fewer species. This is observed everywhere except in the eastern Indian Ocean, where everything is complicated by the merger of two major ocean bodies, filtered through a shallow sea environment.
Meanwhile, as the reversal of the Walker Circulation surface winds occurs, the depth distributions of upper ocean tropical water evens out between the two extremes. The primary production patterns reverse, tending to allow more species to invade the eastern oceans, and for those species usually constrained to depths that make them unavailable to surface fishing gear to spend more time near the surface. The western Pacific seine fisheries start to observe more large yellowfin in their catches, as soon as the signature Kelvin wave is released, and the thermocline structure emerges.
To the east, the deepening of the thermocline, and suppression of primary production allows more species access, and deepens the habitat, providing more living space, and making the usually vulnerable tunas less visible, hence less vulnerable to surface fishing techniques. Catch rates tend to decline rapidly, as the signature Kelvin wave arrives, and in situ heating due to night time cloudiness occurs.
The deeper fishing longline tuna gear exhibits generally opposite trends to those experienced by the surface seine gear and must change distribution to enable their gear to be effective - locating deeper, oxygen rich water is their objective.
In extreme situations of ENSO warming the expansion of tropical tuna habitat poleward is notable, and makes available to usually temperate ocean port fisheries some portion of the tropical tunas and associated species such as marlins, dolphin fish (Coryphaena spp.), and some jacks (Seriola and others). These invasions by tropical predators can cause havoc in local fisheries where small pelagics and other coastal species can be preyed upon by these voracious predators species. In cold periods, these same coastal regions tend to develop stronger upwelling, hence more local production, and abundances of secondary predator species from anchovies to jellyfishes can bloom. Beyond the usual local predator populations, toward the poles, and seaward, there are entire arrays of usually excluded predators. Offshore of the eastern boundary currents lie the more tropical and subtropical elements of the oceanic environment. During local warming, and warming associated with transport of the ENSO tropical ocean heat resources, these predator fields are often overlain onto the coastal environment, suppressing upwelling production, and causing the local fauna to submerge. By making vertical dives, the invaders from offshore can devastate local coastal resources.
On the other hand, during extreme cool events, coastal upwelling can become more intense as well as more frequent, tending to transport more of the production offshore, leaving little behind for emergent fish larvae, or birds, and can in this fashion also devastate local production. It is, in fact, the balance between extremes, the transitional processes, and the dynamic interactions between the various ecological components, and compartments that is important, and links these systems into an interactive, sustainable whole. As I have stated at another time in the La Niña Summit, the worst things that happen during any one phase or stage of the ENSO cycle is good news for another set of participants. The physical dynamic is the reset button in the patterns of ecological winners and losers.