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In the Polish costal zone of the Baltic Sea two similar looking gobies – sand goby and common goby occur. In quantitative investigation both species are often taken as an one, because of difficulties in distinguish them. There are a few methods to differentiate sand goby and common goby - body pigmentation, shape of head and pigmentation of a spleen (Ławacz 1965). During the investigation it was proofed that compared species differ from each other also in biological characteristic. In the Gulf of Gdańsk spawning-season starts for sand goby in March and ends in September, for common goby it lasts from March to August. They have different sex ratio, but they have similar range of a HSI and GSI coefficients. Those differences and similarities are important for future investigations because they show that sand goby and common goby should be separated in studies
The reproduction aspect of the biology of Pomatoschistus minutus was investigated on fish collected in the Gulf of Gdańsk from March to October 2002. The study zone was from the shore to a depth of about 1 m. The sand goby is common in the investigated area from July to October, but it migrates to deeper water in the colder months. The highest numbers of individuals were observed in September. The spawning season of the sand goby in the Gulf of Gdańsk is from April to August. It is so long because this species is batch spawner. The study confirmed a dependence between the HSI and GSI indexes as well as the dependence between relative fecundity and fish length.
The cycle of gonad development and related changes in the length structure and spatial-temporal distribution of ninespine sticklebacks (Pungitius pungitius L.) in Puck Bay were studied. Observations were carried out in the shallow shore zones, as well as in the deeper epipelagic zone of the Bay. Ninespine sticklebacks reproduce in the brackish environment of Puck Bay, breeding in its warmer, inshore zones. Reproduction takes place in the spring and summer months, from April to July. One female spawns several clutches of eggs during one breeding season. The average length of ninespine sticklebacks in Puck Bay was about 40 mm, and the sex ratio in the population was close to 1:1. However, both length structure and sex ratio were subject to local and periodic variations, resulting from possible breeding-related territorial divisions. Higher gonadosomatic indices in females in early spring represented the transition of fish to the advanced vitelligenous phase. The lowest GSI of males during the breeding season indicated the termination of spermatogenesis. The completion of spawning in August started a new process of gonad restoration to prepare the fish for the next breeding season.
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