Potential Impacts of Marine Seismic Survey on Marine Life and Fish Catch Rates in Tañon Strait

Lourdes S. Arciaga

Bureau of Fisheries and Aquatic Resources Region VII
Cebu City, Philippines

Abstract

This study was a review of the potential impacts of marine seismic survey in Tañon Strait on marine life and fish catch rates based on secondary data and actual field records. Basic facts and figures would show that the potential impacts were not as alarming as perceived.

The marine seismic survey done on the northern part of the Strait has avoided the more critical habitat of marine mammals and major sink area for planktonic organisms that are located on the southern part. Studies have proved that lethal impacts of airgun pulses are confined to short distances and affect minimal percentages of plankton and fish populations. Population structures of fishes and marine mammals found in the area showed that these species are generally tolerant and capable of avoiding lethal effects from airgun sound pressures. Most fishes caught in the Strait such as tunas, mackerels and scads are powerful swimmers and migratory species. All marine mammals found in the area belong to the toothed-whale group that is also known as fast swimmers and more exposed to anthropogenic activities. Invertebrates such as squids and shrimps are likely the least affected since they lack swim bladders, that most fishes have. Swim bladders may get ruptured when fishes get exposed within the lethal range from airgun pressures. Impacts to fish recruitment would be minimal due to limited spawning in the area, while fish eggs and larvae are being supplied by the shallower Visayan Sea in the north. Concerns are focused on some marine organisms on their critical life stage like young marine mammals and fish spawners. Reported fish kills could be an indicator of some pathological impacts on bottom-dwelling, territorial and less mobile fishes thriving along steep continental slopes of the Strait.

Analysis on the catch and effort data (CPUE) from municipal fisheries in northern Tañon Strait using drift gill net and bottom set gill net showed that, the seismic survey has caused behavioral disturbance on commercially important fishes, but this did not significantly affected the daily fish catch of municipal fishermen. No significant change was found on the CPUE and average daily catch of drift gill net fishermen. CPUE for bottom set gill net fishermen, on the other hand, registered a 226 % increase in catch in most boat-trips and zero catch in some boat-trips during the period of intense seismic shooting at the central part of the Strait (about 100 kms, away). Fishes caught using bottom set gill net during this period were mostly composed of mid-water dwelling Indian mackerels, an indication that avoiding fishes were swimming low closer to the bottom. In general, impacts of the seismic survey to fish catch rates are minimal, even favorable to some fishers, and temporary, as evidenced by the return to normal levels of CPUE and average daily catches few days after the seismic survey.