| Oratosquilla oratoria is a typical commercially exploited stomatopod species that is widely distributed in the coastal waters of the Northwest Pacific. It also plays an important role in structuring the benthic communities and can significantly affect marine nutrient cycling and energy flow through sediment bioturbation. Few studies have been published on the early life stages and recruitment of O. oratoria in natural waters, particularly in Chinese coastal waters. Moreover, despite ample research on the egg and larval development, growth, feeding, reproductivity, and spatial distribution of this species, little is known regarding the habitats required for juvenile O. oratoria when they settle down to the bottom after the planktonic larval stage. In this context, one objective of the present study was to investigate the seasonal distribution of juvenile O. oratoria and its influencing factors. The other objective was to identify the early recruitment pattern of O. oratoria in the Bohai Sea based on the seasonal pattern of its juvenile occurrence and published literature on the spawning, embryonic and larval development, and growth of this species.
Data used to achieve these objectives were mainly collected from eight monthly cruises of bottom trawl surveys for fisheries resources in spring (i.e., March to June) and summer to fall (i.e., August to November) of 2017. The same pair of bottom trawlers was used in all surveys, following consistent sampling protocols. O. oratoria catches were counted and weighed in number and biomass on the spot at each station, along with the depth, geographic coordinates, date, and time at the beginning and end of each tow. In addition, 50 O. oratoria individuals were randomly sampled to measure their biological characteristics (e.g., total length, weight, sex, feeding status, and fecundity) at each station; at certain stations, all individuals were measured when fewer than 50 O. oratoria were caught. The smallest and largest individuals were selected at each station to record their total length and weight when any O. oratoria was caught at a station. Moreover, surface-to-bottom temperature and salinity profiles were obtained with a SEABIRD CTD within a few minutes of the bottom trawlers moving away from the end of trawling at each station.
The occurrence of juvenile O. oratoria (<60 mm total length) was determined based on the total length of the smallest individuals at each station. Subsequently, their occurrence over space, occurrence probability, and sample size were analyzed by season and month. Generalized additive models (GAMs) were used to identify significant environmental factors affecting the occurrence of juveniles separately in spring and summer to fall. Environmental factors considered in the model included depth, bottom temperature (BT), bottom salinity (BS), and five sediment-related variables. Depth, BT, and BS were measured simultaneously with the bottom trawl surveys. The five sediment variables were classified into two groups: two content variables separately representing the percentages of sand and silt and the other three representing statistical parameters of grain size distribution (i.e., mean grain size, skewness, and kurtosis) in the surface substrates (0–3 cm). The spatial distribution of these sediment factors was derived from recent literature on the grain-size distribution of surface sediments in the Bohai Sea (Yuan et al, 2020). A pseudo-stepwise procedure was used to identify significant environmental factors for optimal GAMs separately for spring and summer to fall based on the Akaike Information Criterion (AIC). Furthermore, the early recruitment pattern of O. oratoria in the Bohai Sea was elucidated in terms of the timing of early life stages and recruitment through combined analysis of historical literature on the seasonal patterns of juvenile occurrence and published literature on the spawning, embryonic and larval development, and growth of this species.
Spring occurrences were severely limited by bottom temperature and sediment mean grain size and skewness; most of the juveniles occurred in Laizhou Bay and its adjacent waters, with a few others observed in the Liaodong Bay and coastal waters northwest of Dalian City. Juvenile occurrence frequency was the highest in April, followed by May, during spring. Additionally, juvenile O. oratoria occurred throughout the survey area within the Bohai Sea in September and October, with the highest juvenile sample size recorded in November, following the same sampling protocol. The occurrence distribution in fall was shaped by bottom temperature, bottom salinity, and sediment sand content, although these three factors only explained 15.5% variance in fall occurrence. Therefore, temperature and sediment elements have greater effects on juvenile O. oratoria occurrence in spring than in summer. Furthermore, the recruitments of O. oratoria tended to be multi-seasonal in the Bohai Sea. The spawning season may last from May to September. Fertilized eggs released from May to July would develop into juveniles from July to September and then grow to 50 mm in total length from September to November. Eggs released in August and September would develop into juveniles from October to November, stop growing in December to March when the temperature drops below 8℃, and then grow to large juveniles in April and May. The validation of such timings requires further studies of the monthly or seasonal trends of O. oratoria larval abundance and distribution in the Bohai Sea. |
