Length-weight relationships and condition factors of 24 mesopelagic fishes from the South China Sea

Mesopelagic fishes are one of the most abundant marine organisms and have a wide distribution in the world’s oceans, thereby playing a central role in global marine ecology. We determined the length-weight relationships (LWRs) of 24 mesopelagic fish species, representing 11 families and 6 orders, inhabiting the South China Sea (SCS), and also assessed their growth types and various condition factors, using samples acquired from midwater trawls at depths of 70–750 m during eight cruises by a single fishing vessel in 2015–2018. Based on 894 specimens, the LWRs of the species were calculated for the first time. The values of the b coefficients in the LWRs ranged from 2.2669 (for Sigmops gracilis ) to 3.5471 (for Ichthyococcus ovatus ), except 2 species exhibited b values less than 2.5. The 95% confidence intervals of the a and b values were tested to identify the growth patterns: 10 species exhibited isometric growth, 7 species negative allometric growth, and 7 species positive allometric growth. The means of Fulton’s condition factor ( K c ) ranged from 0.234 ± 0.046 (for S. gracilis ) to 3.318 ± 0.781 (for Caristius macropus ); the allometric condition factor ( K a ) from 0.149 ± 0.014 (for Scopelosaurus hoedti ) to 6.758 ± 1.162 ( C. macropus ); and the relative weight condition factor ( K n ) from 1.000 ± 0.090 (for Scombrolabrax heterolepis ) to 1.023 ± 0.208 (for Macropinna microstoma ). The K c values of fish in the families Opisthoproctidae and Myctophidae were mostly > 1, denoting species tending to have a relatively short, compressed, and deep body; conversely, fish with K c values of < 1 tended to have an elongated body shape or a long, narrow tail. The K a values were > 2 for 5 species, indicating the best condition compared with the other species; the values of 9 species ranged from 1.132 ± 0.135 to 1.938 ± 0.276, indicating a better condition; and the values of 10 species were < 1, indicating a poorer condition. The K n values ranged from 0.983 ± 0.137 to 1.031 ± 0.249, whereby the results for 22 species reflected a good growth condition ( K n > 1), with the values of just two species being scarcely less than 1.000. In conclusion, these data will be helpful in future studies of mesopelagic fishes and provide basic biological data useful to fishery assessments and conservation of the mesopelagic fish resources in the SCS.