Fish-based index reveals seasonal and spatial differences in Hainan’s coastal health

A fish-based index of biotic integrity (F-IBI) developed for the inshore waters of Hainan Island identified clear differences in ecological condition between the island’s northwestern and southeastern sectors, with most sampled stations rated good or fair and the highest scores recorded in the southeast. The assessment, based on bottom-trawl surveys conducted in May and September 2022, found mean F-IBI scores of 48.16 ± 8.99 in spring and 56.30 ± 7.77 in fall. A one-way analysis of variance showed a highly significant difference between the seasons (F = 23.498, P < 0.01), indicating higher index scores in fall, although the study did not establish that fishing or any other single factor caused the seasonal change.

The researchers surveyed 50 stations across Hainan’s inshore waters, using a single one-hour daytime otter-trawl haul at each station during each of two campaigns. The vessel, Guibeiyu 69068, had a 436 kW main engine, a 37.7 m headrope and a 20 cm codend mesh, and trawls were conducted at approximately 3 knots. Fish were sorted, counted, weighed to the nearest 0.1 g and measured on board; specimens that could not be identified to species level were frozen for laboratory identification. Taxonomic status was checked against recent ichthyological literature and authoritative species listings. The surveys captured 363 fish species from 24 orders, 114 families and 226 genera. Ray-finned fishes accounted for 347 species, or 95.59% of the total, while 15 elasmobranch species represented 4.13%.

Perciformes was the most species-rich order, with 181 species across 56 families and 107 genera, or 49.86% of all species recorded. Scorpaeniformes followed with 39 species, Pleuronectiformes with 34 and Anguilliformes with 22. Six species occurred at more than 60% of survey sites: Acropoma japonicum, Decapterus maruadsi, Saurida tumbil, Saurida undosquamis, Champsodon atridorsalis and Photopectoralis bindus. The seasonal catch composition also differed. Acropoma japonicum had the highest spring index of relative importance (IRI), at 3013.82, followed by Decapterus maruadsi at 1475.74, while Photopectoralis bindus led the fall ranking with an IRI of 730.84. IRI combines each species’ numerical abundance, biomass contribution and frequency of occurrence across sampling stations; values of 1000 or more are classified as dominant.

Dietary and thermal groupings described a fish assemblage dominated by omnivores and warm-temperate species. Omnivorous species numbered 142, representing 39.12% of the total, while 138 benthic-animal-feeding species accounted for 38.02%. Piscivorous species comprised 48, or 13.22%, and zooplanktivorous species 33, or 9.09%; two species were classified as generalists. Warm-temperate fishes made up 335 species, or 92.29%, compared with 25 warm-water species and three cold-temperate species. Most species, 323 or 88.98%, were recorded as spawning floating eggs. Ecologically, 244 species were classified as demersal or near-demersal, 61 as pelagic and 58 as reef-associated.

Community analyses consistently separated the stations into two broad spatial groups in both seasons. In spring, one group was concentrated in the northwestern waters of the Beibu Gulf and Qiongzhou Strait, areas described as having dense human populations, heavy marine traffic and substantial anthropogenic disturbance. The other occupied southeastern waters with more open marine conditions, several protected areas and comparatively less human interference. Non-metric multidimensional scaling supported the cluster pattern, while analysis of similarity detected highly significant differences between the two groups in spring (R = 0.527, P < 0.01) and fall (R = 0.72, P < 0.01). Although fall clustering retained the same general spatial distribution as spring, the non-metric scaling stress was below 0.2, which the authors described as offering some explanatory significance.

To construct the Hainan-specific index, the team began with 25 candidate indicators covering species composition and abundance, trophic guild, thermal tolerance, reproductive guild and habitat preference. A distribution screen removed indicators that occurred at fewer than 10 species across all stations, had proportional variation below 10% or contained more than 90% zero values. A second screen eliminated six indicators whose reference-site and disturbed-site medians overlapped within the interquartile range. Pearson correlation analysis then identified ten retained core indicators: number of fish species, the Shannon–Wiener index, Margalef species richness, percentage of Perciformes, percentage of piscivorous fish, percentage of benthivorous fish, and four habitat-related measures covering demersal sandy-mud-bottom fish, near-bottom fish, mid-upper-water-column fish and reef-associated fish.

Each indicator was standardized using the ratio method, with values expected to decline under disturbance scaled against the 95th percentile across all stations and values expected to increase scaled against the maximum and fifth-percentile values. The F-IBI score for each station was the mean of its standardized indicator values. The health threshold was set at the 25th percentile of scores among the reference stations, which were five sites considered relatively well preserved and minimally disturbed: S16, S19, S32, S45 and S50. The resulting classification placed scores of 68.58 or above in the healthy category, 51.44 to 68.58 in good, 34.29 to 51.44 in fair, 17.15 to 34.29 in poor and below 17.15 in bad. No sampled site was classified as extremely poor in either season.

Individual spring F-IBI scores ranged from 25.50 to 62.89, whereas fall values ranged from 41.01 to 70.06. Most stations fell within the good or fair categories in both surveys, with the highest spring score recorded at S45 and the highest fall score at S16. The southeastern stations generally performed better than those in the northwest and Qiongzhou Strait. The researchers associated the lower northwestern scores with factors including heavy fishing activity and marine traffic, port construction, tourism-related coastal development, industrial waste discharges and coral bleaching, while southeastern sites near nature reserves generally experienced less disturbance and retained more intact habitats. These observations describe spatial associations rather than experimentally isolated causes, and the authors noted that circulation, seabed topography and the distribution of food resources may also help explain community differences.

The seasonal increase followed the island’s May-to-August fishing moratorium, with the fall survey conducted in early September, and the authors suggested that a period of reduced fishing pressure may have allowed resources to recover. They also emphasized that physical and chemical variables—including temperature, salinity and dissolved oxygen—can influence F-IBI scores, so the observed increase cannot be assigned to the moratorium alone. The study nevertheless offers a site-specific framework for comparing fish-community integrity across Hainan’s heterogeneous inshore environment. It also recognizes that the ten-indicator system is preliminary: trophic and thermal measures can vary with interannual conditions and extreme climate events, while the same species may have different life histories across marine regions. Further revision using more extensive biological and environmental data will therefore be needed to determine how reliably the index can track future changes in ecosystem health and guide fisheries management around Hainan Island.

Subject of Research: A preliminary fish-based index for assessing ecological health in Hainan Island’s inshore waters

Article Title: Evaluation of ecological health of Hainan Island inshore waters in the South China Sea based on preliminary fish biotic integrity index ( Actinopterygii and Elasmobranchii )

Article References: Yu, P., Luo, Z., Wang, L., Liu, Y., Huang, Y., Shan, B., Yang, C., & Sun, D. (2024). Evaluation of ecological health of Hainan Island inshore waters in the South China Sea based on preliminary fish biotic integrity index (Actinopterygii and Elasmobranchii). Acta Ichthyologica et Piscatoria, 54, 221-233. https://doi.org/10.3897/aiep.54.130966

Image Credits: AI Generated

DOI: 10.3897/aiep.54.130966

Keywords: Hainan Island, coastal ecology, fish biotic integrity, fishery resources, South China Sea, marine biodiversity, seasonal variation, ecological assessment, Evaluation, ecological, health, Hainan