Water quality evaluation from sea trout spawning grounds in boreal lowland rivers: potential stressors for the developing larvae

The Baltic Sea hosts populations of sea trout (Salmo trutta L.) which have historically used many Lithuanian watercourses for spawning and juvenile recruitment. Knowledge about the spawning habitat quality is of practical importance in implementing effective conservation and restoration measures to support the freshwater-bounded stages. The aim of the present work is to identify the main environmental stressors characterizing the final part of the incubation period of the anadromous brown trout. This is achieved by evaluating the hyporheic water quality of spawning areas in three Lithuanian rivers characterized by different land-cover settings. On a spatial level, hyporheic water held higher values in conductivity and ammonium, signalling high trophic conditions, associated with those areas holding higher shares of urbanized and agricultural features. On a temporal level, during the final incubation phase, in all the three watercourses temperature and oxygen values approached ranges that are associated with reduced survival of intragravel stages. Overall, low hyporheic oxygen values recorded towards the last incubation phase may be further worsened by rising temperatures. Such a condition not only decreases oxygen dissolution, but may also increase the metabolic activity of salmonid larvae, potentially making them more vulnerable to oxygen sagging. Since no embryo and sac fry survival were measured directly, the results only suggest which physicochemical conditions may be more associated with poor incubation success.