Capillaria Moravec, 1987, is a genus of thin, threadlike roundworms infecting various hosts, including fishes. Fifteen species have been described from freshwater, brackish-water and marine fishes, primarily based on morphological characters. However, species identification remains challenging, and no molecular information is currently available for previously described Capillaria species. Utilizing morphological, scanning electron microscopy, and molecular evidence, we describe Capillaria (Procapillaria) nubthaiensis Chan, 2025 as a new species of capillariid infecting the commercially important fish Scomberomorus commerson from the Gulf of Thailand. Examination of 30 male and 30 female specimens revealed key morphological features distinguishing Capillaria nubthaiensis sp. nov. from other members of Capillaria (Procapillaria). These morphological characters include a unique female vulva that appears slightly protruding and forms a ‘knob-like’ shape appendage and a small papilla near the base of each male lobe with another pair of small papillae near the anus. Molecular phylogenies obtained from the nuclear 18S ribosomal RNA and the mitochondrial cytochrome c oxidase subunit I genes also support the distinction from other species in the family Capillariidae. The discovery of C. nubthaiensis sp. nov. represents the first record of a Capillaria species infecting S. commerson, and it is also the first to be recorded in Thailand’s marine waters. Alongside molecular information, this study contributes vital information beneficial for the taxonomy and biodiversity of capillariids in marine fishes.

In response to perceived, but unknown variation among the size selection of narrow-barred Spanish mackerel (Scomberomorus commerson) by artisanal gillnetters off Iran, and the need for such data to control exploitation as a precursor to balanced harvesting, the effects of two common mesh sizes (130 and 140 mm stretched mesh opening made from multifilament twine) on catches were investigated over one fishing year (nine months encompassing autumn to spring). Both mesh sizes mostly caught S. commerson at fork lengths (FL) larger than mean sizes at maturity (>67 cm), with the mean size selection incrementally increasing in the 130-mm mesh gillnet from autumn, and especially during spring. The greater selection occurred concurrent with an increasing condition factor (CF) among S. commerson, which typically spawn in late spring/early summer. Conversely, the relative size-selection of the 140-mm mesh gillnet decreased in spring, attributed to increasing CF precluding the capture of larger fish. Such seasonal variation in size selection might be countered by increasing mesh size to ~145 or 150 mm in spring. However, the existing 140-mm mesh might positively affect stock biomass by allowing larger, more fecund fish to avoid capture during spawning. The data support the strong influence of biological and environmental factors on gillnet size selection, which might also extend to other migratory, pelagic species.

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.