Assessing the reliability of catch and size statistics estimated using electronic monitoring data collected on Seychelles flagged tuna purse seiners

Fisheries data forms the basis of fisheries management, simply because without these data it is practically impossible for fisheries managers to know the status of fish stocks, let alone manage them. Having an economy highly dependent on fisheries, Seychelles together with a Spanish organisation of purse seine fishing companies operating in the Indian Ocean, the Producers’ Organisation of Large Tuna Freezers (OPAGAC), initiated a process of testing technology-based at-sea monitoring systems to better manage the tropical tuna fishery. The Satlink Electronic Monitoring System (EMS) was tested during a seven-month period in 2016 on two Seychelles flagged tuna purse seine vessels. Data collected using EMS was compared to data collected from oversampling of catches in port from selected fish wells. Total retained tuna catch, catch by species and size distribution were compared between the two methods. Results indicated that EMS was more reliable for estimating total retained skipjack tuna (Katsuwonus pelamis) and yellowfin tuna (Thunnus albacares) as well as for estimating proportions of the total catch by species and proportions of the total catch by commercial size category of bigeye tuna (Thunnus obesus). However, EMS was less reliable for estimating total retained bigeye tuna and assessing fish size distributions. EMS also failed to provide accurate estimates for total retained bycatch. This was due to the set-up of the cameras on the lower deck, that prevented dry observers from obtaining accurate estimates of the number of bycatch retained in each fish tank. Overall, EMS seems capable of producing reliable catch estimates, however, both system configuration and data review protocols require further adjustments in order for EMS to be able to obtain more precise and accurate estimates of catch and size