Abstract:

Aim: Nekton surveys in coastal waters can enrich our understanding of species diversity, assess the status of local fishery resources and variations in their trends, and provide the fundamental data that needed for effective management. The aim of this study was to understand nekton species composition, density, and community structure in coastal waters of eastern Fujian Province.

Methods: Bottom trawl surveys were conducted at eight stations in coastal waters of eastern Fujian Province to collect nekton samples in spring (April) and autumn (October) of 2021. Fishes, crustaceans and cephalopods were identified to species level if possible. The index of relative importance (IRI), Shannon-Wiener diversity index based on number (H'_N) and weight (H'_BW), taxonomic diversity indices (taxonomic diversity, Δ; taxonomic distinctness, Δ^*; average taxonomic distinctness, Δ⁺; variation in taxonomic distinctness, Λ⁺) and densities (i.e. abundance and biomass) were calculated. The proportions of the number of juvenile fishes were estimated based on length measurements and the lengths at fish sexual maturation were obtained from Fishbase (www.fishbase.org). Abundance biomass comparison curve (ABC curve) of nekton and different taxonomic groups (fishes, crustaceans and cephalopods) were also evaluated. Non-metric multidimensional scaling (NMDS), similarity percentage analysis (SIMPER), and biota-environment matching (BIOENV) were used to describe the characteristics of the nekton community structure and the relationships with five environmental factors (temperature, dissolved oxygen, salinity, pH and water depth). The differences in the nekton community between spring and autumn were also tested statistically.

Results: A total of 147 nekton species, including 82 fishes, 57 crustaceans, and 8 cephalopods, were collected. Eleven dominant species (IRI ≥ 500) were identified, including Harpadon nehereus, which had the highest IRI values (IRI > 4,000) in both spring and autumn, and was the only species that dominated in both seasons. The dominant species were generally small-sized and low-valued in spring, and large-sized and high-valued in autumn. The Shannon-Wiener diversity index based on weight (H'_BW) in spring was significantly lower than that of autumn (P < 0.05). The low average taxonomic distinctness (Δ⁺) in spring indicated that the taxonomic relationship of nekton species was closely associated. The average densities of nekton were 16.9 × 10³ ± 14.5 × 10³ ind./km² and 474.0 ± 362.0 kg/km² in spring, which were significantly lower than those in autumn (240.8 × 10³ ± 178.6 × 10³ ind./km² and 2,587.5 ± 1,495.5 kg/km²) (P < 0.01). The number of nekton species, the H'_BW and the proportions of the number of juvenile fishes in spring were significantly lower than those in autumn (P < 0.05). ABC curves suggested that the nekton communities in coastal waters of eastern Fujian Province were under a disturbed-even stressed-condition, especially in autumn. The nekton community structure was significantly distinct between spring and autumn (P < 0.01). The community structure was closely related to the combination of temperature and dissolved oxygen factors according to the BIOENV results (r = 0.548, P < 0.01).

Conclusion: This study revealed that the number of nekton species in coastal waters of eastern Fujian Province was relatively high, irrespective of only two seasonal surveys, compared to other coastal areas in Fujian. Based on comparisons of the average density and the average body weight of nekton in 2007-2021 in the same area or other waters of Fujian, the fishery resources in coastal waters of eastern Fujian Province showed a positive trend. As these surveys were conducted before and after the national summer fishing moratorium, long-term monitoring can be applied to evaluate the effectiveness of the fishing moratorium regulation in the future.

Key words: Fujian waters, nekton, community structure, diversity, density, summer fishing moratorium