Mono- vs multifilament gillnets: effects on selectivity of narrow-barred Spanish mackerel Scomberomorus commerson in the Persian Gulf

Abstract

Iranian coastal fishers targeting narrow-barred Spanish mackerel (Scomberomorus commerson) recently replaced their historical multifilament gillnets with those made from monofilament, evoking management concerns over potential increases in catch-per-unit-of-effort. During 20 fishing days, we compared catches from replicate surface-set gillnets that were identical in terms of mesh size (140 mm stretched opening), length (180 m), depth (30 m), hanging ratio (0.56) and spatio-temporal deployment, but had different materials: multifilament (1.8-mm diameter twisted twine) vs monofilament (0.8-mm diameter twine). Compared with the multifilament gillnet, there was a trend of greater catches (up to 1.3×) of S. commerson and another retained species, mackerel tuna (Euthnus affinis), along with one discarded species, giant catfish (Netuma thalassina) by the monofilament gillnet. However, statistical significance was restricted to E. affinis catches and a bias towards smaller S. commerson. These differences were attributed to species-specific catching mechanisms within gillnet material, with larger S. commerson retained by their teeth in the multifilament and all E. affinis more securely retained by their deeper bodies in the monofilament. Gillnet materials require regulation to preclude excessive effort on fully exploited stocks of species such as S. commerson.

Effects of season and mesh size on the selection of narrow-barred Spanish mackerel, Scomberomorus commerson in the Persian Gulf artisanal gillnet fishery

“…Possible solutions for maintaining consistent size selectivity throughout the entire fishing year might encompass some of the key technical factors known to affect gillnet catches, including reducing hanging ratio (Gray et al ., 2005), increasing mesh size (Hamley, 1975), or alternative materials (Eighani et al ., 2020). For example, several studies have revealed negative relationships between gillnet hanging ratios and selectivity or efficiency (Gray et al ., 2005), including for similar-shaped pelagic species (Samaranayka et al ., 1997).…”

“…This is especially the case in Iran, where artisanal gillnetters target S. commerson for nine months each year during the species’ regional migration from October (autumn) to June (spring/summer), after which fishing effort shifts to other species (Eighani et al ., 2020). The sizes of S. commerson caught by Iranian gillnetters typically range from 50 to 120 cm FL and encompass means at 50% maturity of ~67 and 80 cm FL for females and males, respectively, or 1–2-year-old fish (Roa-Ureta et al ., 2019; Eighani et al ., 2020). Peak catches often coincide with spawning during late spring (May and June) (Claereboudt et al ., 2005; Roa-Ureta et al ., 2019).…”

“…Historically, Iranian fishers have used variable-length multifilament polyamide (PA) gillnets up to 30 m deep and made from different mesh sizes, but often 130–140 mm stretched mesh opening (SMO; Eighani et al ., 2020). While there have been some studies describing the relative catches and selectivities of regional gillnets used to target S. commerson (Hosseini et al ., 2017; Eighani et al ., 2020), this work is limited to discrete spatio-temporal scales. Less is known about the effects of longer-term interacting technical, environmental and biological factors implicated to influence the performance of gillnets more broadly (reviewed by Hamley, 1975).…”

“…There is a need to determine an appropriate mesh size and/or configuration that reduces the capture of juveniles while retaining adult S. commerson to achieve sustainable utilization of stock. One important consideration is the choice of netting material since Eighani et al (2020) noted a lower size selectivity for S. commerson in gillnets with the same mesh size but made from monofilament rather than multifilament twine.…”

Effect of baiting gillnets in the Canadian Greenland halibut fishery

“…Length of capture was analysed for each species by comparing the proportion retained from the catch between treatments in each length class following the methods of Holst and Revill (2009) and Eighani, Bayse, Paighambari and Broadhurst (2020). The logit (baited/baited + non‐baited) of the catches‐at‐length was estimated by low‐order polynomial GLMMs (degree 0–4), and the data were fitted as a quasi‐likelihood model within a binomial distributed model using the glmmPQL function.…”