Body size is a basic biological characteristic in fish populations and can reflect individual physiology as well as changing environment conditions. Slight variabilities in some biological parameters may result in complex ecological effects, and affect food web link intensity in trophic cascades. However, the spatial and temporal heterogeneity of size composition within populations have often been ignored in many studies of fish biology. We use Fang's gunnel () in Haizhou Bay as an example for studying the variability of body size on an annual scale. is a low trophic fish and plays an important role in the food web and ecosystem of the Yellow Sea, with increasing dominance in Haizhou Bay. We collected annual bottom trawl surveys in Haizhou Bay in the spring and fall from 2011 to 2016. We used a range of statistical methods, included variable coefficient, covariance analysis, two-sample -test, and Pearson correlation analysis to study the population size composition, length-weight relationship, and relative fatness of in this area. We analyzed the annual and seasonal variability as well as the spatial distribution of body length and relative body mass. The results showed that had multiple age structure in Haizhou Bay. Their length frequency distributions were multi-modal and skewed, with the majority of captured individuals aged 2-3 years. Statistical analysis indicated that there were remarkable temporal and spatial variations in the average size and the parameters of body length-weight relationship of <0.05). The average body length and relative fatness tended to be higher during autumn surveys than spring surveys. Variation in relative fatness was greater between seasons than among years. In spring, the spatial distribution of body length and relative fatness was larger in the southwest area than the northeast area of the bay. A -test on the body length-weight relationship showed that the allometric growth patterns of were generally positive. Correlation analyses between benthic water temperature and the length-weight relationship showed that temperature had a substantial influence on the relative fatness, body length-weight relationship, and mean body length. The spatiotemporal variability of fish size and other parameters may be attributed to their feeding intensity, maturation, fishing pressure, and environmental and habit variation, and is also likely to reflect changes in the fishery ecosystem. We suggest that the spatiotemporal variability of population size composition should be fully considered in fisheries resource management, as these basic parameters can contribute significantly to fishery ecosystem modelling and management strategy evaluation.