Effects of polluted groundwater on chum salmon (Oncorhynchus keta) survival and body size.

This study reports the effect of spatial variation in hyporheic water, partially influenced by urban-polluted groundwater, on the early life stage of chum salmon (Oncorhynchus keta) in the Toyohira River, Northern Japan. We hypothesized that increased groundwater influence would reduce the survival rate and body size of O. keta due to the combined effects (i.e., growth retardation effects) of chemical toxicants, low dissolved oxygen (DO), and high winter temperatures. Experimental tests were conducted in field and laboratory conditions to address the difficulties associated with field observations of fry emergence during snowmelt floods in spring and to examine the independent effects of water pollution in groundwater in relation to temperature and DO. Artificially fertilized eyed eggs, alevins, and fry of O. keta were monitored for several months with varying exposure to groundwater from winter to early spring. We noted that groundwater affected the fish by reducing their size and weight by >10% and by increasing their mortality in both tests. Moreover, independent effects of water pollution were identified in the swim-up fry stage in laboratory experiments, along with growth-retarding effects from warmer groundwater temperatures. Not all factorial combinations of potentially confounding factors were tested rigorously, and the specific toxicants are unidentified, leaving questions about how groundwater pollution affects Salmonidae fish. Immediate concerns regarding the current water quality (including DO) of hyporheic water associated with groundwater influence are low because no detrimental effects on survival were detected in the field. Nevertheless, spawning grounds formed in areas with high exposures to polluted groundwater require continuous management attention due to potential risks associated with low DO levels. Additionally, pollution-induced growth patterns could pose a risk of size- or weight-dependent mortality at the swim-up fry stage and in early juveniles.