Growth was studied with the aid of back calculations, while Lagler's method was applied to following the length -weight relationship. Investigations on feeding of the Szczecin Firth Aspius aspius were restricted to food composition and occurrence frequency of the food items. Additionally, growth and feeding of the Szczecin Firth Aspius aspius were compa red to those of the species occurring in other water bodies.
Spermiation in 3-year-old sterlet, Acipenser ruthenus (L.), males maintained under warm water conditions was induced by intramuscular injection of either (i) (D-Ala 6 )-GnRH-ProNHEt (Kobarelin); (ii) mammalian GnRH analogue1metoclopramide (Ovopel); or (iii) human chorionic gonadotropin (Biogonadyl). The volume of milt, sperm concentration and motility were measured. A higher percentage of spermiating males was obtained after Kobarelin or Ovopel treatment (81.8% and 77.7% respectively) in comparison with ¢sh treated with Biogonadyl (40.0%). However, only stimulation with Ovopel guaranteed motile spermatozoa in all spermiating males. Moreover, treatment with Ovopel resulted in the highest average milt volume, sperm concentration and motility. The average total number of motile spermatozoa (milt volume  sperm concentration  sperm motility) per individual was 0.99  10 9 , 5.31  10 9 and 0.02  10 9 after stimulation with Kobarelin, Ovopel or Biogonadyl respectively. Our data indicate that Ovopel is a good stimulator of spermation in sturgeons. Moreover, the time of sexual maturation could be signi¢cantly reduced in sturgeons maintained in cages under warm water conditions.
The whitefish (Coregonus lavaretus) of the Lake Miedwie was studied. Based on gill raker counts, three f onns of the species are discerned: lavaretus, maraena, and generosus. The maraena fonn individuals showed the highest growth rate. The other two forms grew more slowly, their growth rates being similar to each other. The maraena andgenerosus fonns fed on both plankton and benthos, while lavaretus consumed betnhos only;
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