Kenya’s coastal gillnet fishery: tradeoffs in mesh-size regulations:

Our new study led by Kennedy Osuka and published in African Journal of Marine Science has found catches in small mesh sizes comprise high proportions of juveniles

while large mesh sizes capture species ranked as Near Threatened and Vulnerable on the IUCN Red List. On a more positive note, medium mesh sizes are associated with low trade-offs between ecological impact and fishery returns and are recommended for artisanal fisheries.

The issue

Gillnets are widely used gears among Kenya’s coastal artisanal fishers. However, their mesh sizes are inadequately monitored or regulated. Policy recommendations to manage the gear are therefore needed.

What we did

We undertook deployment and catch surveys to collect effort and catch data from 147 gillnet fishing trips. The impacts of gillnets of seven stretched-mesh sizes were then evaluated through comparative analysis of species-related metrics and catch per unit effort (CPUE).

What we found

  • Three mesh-size groups were identified from catch-composition data and comprised small (1.3, 5.1 and 7.6 cm), medium (10.2 and 15.2 cm) and large (20.3 and 25.4 cm) mesh sizes.
  • The dominant species caught in small, medium and large mesh sizes, respectively, were whitespotted rabbitfish Siganus sutor, mackerel tuna Euthynnus affinis and honeycomb stingray Himantura uarnak.
  • Catches in small mesh comprised high proportions of juveniles (61.3 to 74.2%).
  • The medium mesh sizes caught species of mid–high trophic level with high-income returns, displayed moderate CPUE, had the lowest juvenile retention and capture of threatened species.
  • Large mesh sizes captured Near Threatened and Vulnerable IUCN red listed species.
  • Policy recommendations

    For the first time we assessed impacts of different gillnet mesh sizes used in Kenyan coastal waters.

    Pie charts showing the composition of the fish catches, as represented by their IUCN Red List categories, in gillnets of different mesh sizes: (a) 1.3 cm; (b) 5.1 cm; (c) 7.6 cm; (d) 10.2 cm; (e) 15.2 cm; (f) 20.3 cm; (g) 25.4 cm. Percentages are labelled only for categories considered threatened. IUCN Red List categories: EN = Endangered; VU = Vulnerable; NT = Near Threatened; LC = Least Concern; DD = Data Deficient; NE = Not Evaluated
    Of the three mesh-size groups identified, medium mesh sizes were associated with the fewest apparent ecological effects and with moderate fisheries yields. The study recommends the following:

  • Promote the use of mesh sizes of 10.2 cm and 15.2 cm, which capture fast-growing and early-maturing species of mid- to high-trophic-level, and are characterised by a moderate biomass CPUE, low rates of juvenile retention and low catches of Near Threatened, Vulnerable and Endangered sharks and rays.
  • Phase out mesh sizes of 20.3 cm and 25.4 cm due to the high capture of threatened species like sharks and rays.
  • Phase out the mesh sizes of 5.1 cm and 7.6 cm due to their capture of high numbers of juvenile fish and the low biomass CPUE.
  • Temporal and spatial restriction of a mesh size of 1.3 cm to ensure sustainable harvesting of small seasonal pelagic species such as tropical halfbeak Hyporhamphus affinis and herring species of the genus Herklotsichthys.
  • The article can be accessed through the journal page here.

    Full citation: Osuka, K., Kawaka, J.A. and Samoilys, M.A., 2021. Evaluating Kenya’s coastal gillnet fishery: trade-offs in recommended mesh-size regulations. African Journal of Marine Science, 43:15-29.