Increasing Abundance of Silky Sharks in the Eastern Indian Ocean: Good News or a Reason to be Cautious?

Benaya Simeon,Dharmadi Dharmadi,Efin Muttaqin,Irfan Yulianto,Fahmi Fahmi,Ulfah Mardhiah,Muhammad Ichsan,Andhika Prasetyo

doi:10.3390/fishes3030029

Abstract

The silky shark (Carcharhinus falciformis) is the most common shark landed in Indonesian waters, including in the Indian Ocean. However, even though they are frequently caught, there is a lack of information on the population and abundance of silky sharks in the Indian Ocean. Cilacap and Tanjung Luar are two of Indonesia’s shark fishery hotspots, both located on the edge of the Indian Ocean, which is a part of the Indonesian Fisheries Management Area (FMA) 573. The fishers from both places usually have fishing grounds in that particular fisheries management area. This research aims to update the abundance, and to investigate the factors, which influence catches of silky shark in the FMA 573 based on monthly enumeration during 2015–2016. This study found that standardized catch-per-unit-effort (CPUE) or abundance indices of silky shark in the Indonesian FMA 573 increased from 2015 to 2016. We suggested that fish immigration and decreasing fishing pressure may affect the fish abundance. Fisheries management is required to regulate the fishing pressure, such as controlling the number of boats/licenses and the number of trips or fishing gear, as well as initializing spatial and temporal fishing closure.

Highlights

Sharks play an important role within food-webs, and a reduction in the population size of large sharks can initiate trophic cascades through top-down effects [1,2,3,4]
As one of the most common shark species in Indonesian waters, the silky shark dominated the total sharks landed in Tanjung Luar by contributing about 43% in 2012 [15], and more than 30% from 2014 to 2016 [16,17]
We found significantly higher than the catch of the bottom longline

Summary

Introduction

Sharks play an important role within food-webs, and a reduction in the population size of large sharks can initiate trophic cascades through top-down effects [1,2,3,4]. Widespread exploitation and habitat degradation have resulted in a substantial decline in shark populations [1,3,5,6,7]. One of the most harvested species is silky shark (Carcharhinus falciformis), a pelagic shark that is commonly found in the coastal warm-temperate and tropical regions [9,10]. This species is caught by both coastal targeted fisheries and as a major bycatch species in the tuna fisheries. The level of silky shark’s fishing mortality is estimated to have depleted the total stock biomass by 70% from the Fishes 2018, 3, 29; doi:10.3390/fishes3030029 www.mdpi.com/journal/fishes

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Conclusion