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Behavior

The Gulf of Mexico Triangle: Assessing Movements of Large Pelagic Fish Across Multiple National Boundaries

Article

Rooker, Jay R., et al. “Population connectivity of pelagic megafauna in the Cuba-Mexico-United States triangle.” Scientific reports 9.1 (2019): 1663.

https://www.nature.com/articles/s41598-018-38144-8

Background

Large pelagic fishes such as marlin, tunas, and sharks are incredibly valuable for maintaining ecosystem balance as top predators and as highly sought-after species in commercial and recreational fisheries. Yet, many of the species in these groups often undergo vast migrations covering the jurisdiction of numerous countries. Crossing through the coastal waters of numerous countries means that these species are often exposed to different fisheries and management strategies. As a result, overexploitation and incidental bycatch (being caught without being the target species) have become the major barriers for managing theses species. In order to create meaningful management strategies, several multinational initiatives are being designed to preserve marlin, tunas, and sharks.

Figure 1. The six sections of the Gulf of Mexico examined in this article: (1) U.S. Gulf of Mexico including the Florida Keys, (2) Mexican territorial waters (3) High seas areas in the Gulf of Mexico, (4) Cuban territorial waters, (5) U.S. waters in the Atlantic Ocean, and (6) Atlantic Ocean excluding areas 1–5. (Rooker et al., 2018)

In the Gulf of Mexico (GoM), Cuba, Mexico and the United States are developing initiatives to conserve pelagic fisheries. In order to implement effective, sustainable management, it is necessary to have accurate information on the spatial movements of these species throughout the year. Several research groups in the northern GoM have programs that study the growth, feeding, reproduction, and movement in large pelagic fish species. By combining years’ worth of satellite and acoustic tagging data, scientists were able to examine how species of marlin, tuna, and sharks move in the GoM.

Study Goals

The goal of this study was to look at long term trends in the movement of eight species of pelagic fish (blue marlin, white marlin, bluefin tuna, yellowfin tuna, whale sharks, shortfin mako sharks, scalloped hammerhead sharks, and tiger sharks) in the GoM. Species were tagged with several types of acoustic tags and satellite tags including smart position tags (SPOT), pop up archival tags (PATs), and archival implants tags (AITs). While each tag functions differently, these tags could be reliably used to estimate daily positions for individuals. For the purpose of this study, the GoM and surrounding waters were divided into six parts (Fig. 1) to determine how much movement occurred in the waters of Cuba, Mexico, the United Stated, and the high seas or international waters.

Findings

Figure 2. Daily positions for (A) blue marlin, (B) white marlin, (C) Atlantic bluefin tuna, and (D) yellowfin tuna in the Gulf of Mexico. (Rooker et al., 2018)

Figure 3. Daily positions for (A) whale sharks, (B) tiger sharks, (C) scalloped hammerhead sharks, and (D) shortfin mako sharks in the Gulf of Mexico. (Rooker et al., 2018)

Of the eight species of fish tagged in the northern GoM as part of this study, almost all of them passed through the waters of multiple countries. Yet, whether or not a species was considered binational (traveling through the waters of two countries) or trinational (three countries) was highly dependent on the species. Marlin and tunas commonly moved throughout the GoM and entered the waters of all three countries in contrast to sharks where the whale shark was most likely to move throughout the ocean basin. One thing worth noting was the days at liberty for individuals or the number of days that position data was received from the time the fish was tagged. The longer positioning information was received from an individual, the more likely it was to display binational or trinational connectivity (>150 days). Interestingly, the yellowfin tuna, scalloped hammerhead, and tiger shark displayed limited movement outside of the northern GoM. In fact, no yellowfin tuna or scalloped hammerhead was considered trinational, and they visited the fewest regions of the GoM on average of all species tagged (Table 1).

Table 1. Binational and trinational movement information for species based on all individuals with >150 days of daily positions. Including data on regions visited as well. (Rooker et al., 2018)

There are several hypotheses about what drives pelagic fish movements, but in general, large marine predators are thought to undergo long distance movements for feeding opportunities, reproduction, and physiological reasons. All of the species examined in this study use the GoM for reproductive purposes. However, previous studies have shown that species movements can be influenced by the use of coastal and oceanic habitats. Blue marlin, white marlin, bluefin tuna, and whale sharks had daily positions that were primarily in oceanic ecosystems These species also tended to travel the farthest from where the were tagged and crossed the most international borders. Yellowfin tuna, shortfin mako sharks, scalloped hammerhead sharks, and tiger sharks used coastal habitats and were less likely to cross international borders. The authors hypothesize that this may be the result of intricate coastal environments and the presence of resources; readily available resources reduce the need for long distance travel.

Interestingly, for all of the species that had binational or trinational movements, there was a relationship with seasonality. The authors hypothesized that seasonal movements are a way for species to reduce variability in environmental conditions such as sea surface temperature, salinity, dissolved oxygen, and chlorophyll a or the amount of algae. During the spring and summer, blue marlin, white marlin, yellowfin tuna, and whale sharks had daily positions in the northern GoM. In the winter and fall these species moved south into Cuban and/or Mexican territorial waters.

Final Thoughts

Figure 4. Map of international crossing hotspots for marlin and tuna (A) and sharks (B). (Rooker et al., 2018)

Appropriate management of marine resources requires an understanding of a species movements over space and time. Many pelagic species are difficult to manage because they travel across vast distances and the jurisdictions of multiple countries. This creates a unique situation where species management requires multinational, sustainable measures. This study represents a novel attempt to determine migratory pathways in the GoM for multiple large pelagic species. Future similar studies may provide some direction for international management efforts.

Discussion

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