Spawning Potential Ratio (SPR) in the Management of the Swordfish (Xiphias gladius Linnaeus, 1758) in Coastal Waters of Kenya

The swordfish (Xiphias gladius) is a migratory species found in sub-tropical, tropical, and temperate waters. Itmakes significant contributions to food security, nutrition, local income, and revenue for communities and countries. However, there is a lack of comprehensive information and data regarding the biological characteristics and stock status of swordfish in the nearshore marine waters of Kenya. Researchers utilize the spawning potential ratio (SPR), a proxy index commonly employed to determine the health of fish populations. This study applied length-based spawning potential ratio (SPR) to evaluate the reproductive and recruitment capacity of swordfish in the coastal fishery of Kenya. Data used in the analysis was collected from artisanal fishers between August 2015 and December 2016. FISAT (ELEFAN) and the von-Bertalanffy function were employed to analyze growth parameters, mortality rates, and asymptotic length. The SPR was calculated using the tool available at http://barefootecologist.com.au/lbspr.
The study yielded the following results: total mortality (Z) was estimated at 1.13 per year, with natural losses (M) estimated at 0.44 per year and fishing-induced mortality (F) at 0.69 per year. The rate of exploitation (E) was determined to be 0.61 per year. The length at which growth reaches infinity (L∞) was measured at 208 cm, and the growth coefficient (K) was calculated as 0.28 per year. Other findings included to = 0.18, an SPR of 18% (ranging from 12% to 23%), an M/K ratio of 1, an F/M ratio of 1.4, an L50 of 129 cm LJFL, and an SL50 of 87.98 cm LJFL. The minimum size recorded was 68 cm LJFL, while the maximum size reached 234 cm LJFL. The study indicated that the swordfish fishery is predominantly comprised of young individuals, leading to reduction in reproductive and recruitment potential. Therefore, it is recommended to closely monitor the swordfish fishery in Kenya waters and beyond, and consider the application of SPR metrics to inform conservation and management efforts.

Keywords: Kenya, Indian Ocean, Swordfish, Mortality, Spawning Potential Ratio.