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1. Overfishing, exacerbated by illegal, unreported, and unregulated (IUU) fishing, is a serious threat to the conservation of the Caspian sturgeon populations, exposing them to the brink of extinction. This indicates the importance of investigating the causes and eliminating the consequences of the occurrence of IUU fishing.2. This study aimed to determine the barriers to sturgeon conservation, and to evaluate the importance of variables involved in the occurrence of IUU fishing in the southern Caspian Sea using field-and questionnaire-based surveys of 520 Iranian fishers and 40 fishery experts.3. Modelling the data using the Logit regression model indicated that several social, economic, conservation, and fishery-related variables (including fisheries knowledge, fish price, fishing method, fishing time, catch/vessel ownership, conservation importance, and penalty awareness) significantly contributed to the occurrence of illegal fishing. Fishers with poorer fisheries knowledge who owned fishing vessels were more likely to be involved in IUU fishing. In addition, fishers who were less concerned about sturgeon conservation and who used non-standard fishing gear at night had a higher probability of committing IUU fishing. 4. Exploring the opinions of fishery experts through the analytical hierarchy process also showed that economic, social, fishing, and conservation criteria were respectively attributed the highest weights as the contributing criteria to the occurrence of IUU fishing. 5. Overall, close associations were observed between the range of determinants, with the probability of the occurrence of IUU fishing indicating that illegal fishing is a complex event that should be studied in different dimensions because of the involvement of a combination of drivers. The knowledge obtained here can assist the relevant agencies in preventing this widespread problem, and with the practical rebuilding and more efficient conservation planning of sturgeon stocks.

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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.