Less Road Salt = More Eggs

Last month, I shared a photographic journey around the Deer Creek watershed in Maryland with Jim Thompson from Maryland Fisheries Service (Fisheries Service). In this video, Jim explains how they sample for herring eggs in Maryland streams. Today, I wanted to follow-up on that adventure by looking at some of the results of this ongoing study from the annual report [PDF] recently released by the Fisheries Service. For the moment, I thought I would focus on how the herring egg samples are used as indicators of development in the surrounding area.

Road Salt Diagram

For this study, the Fisheries Service examined egg density of different fish species in watersheds that are influenced by different levels of development. Three of the factors compared in this study were:

  • Proportion of samples with herring eggs and/or larvae (aka Pherr)
  • Number of structures per hectare (aka C/ha) between 1970’s and the present— this is a factor that is used to indicate the extent of development in an area
  • Conductivity— this factor estimates the total amount of solids dissolved in water, which is directly related to both water’s ability to conduct electricity (hence conductivity) and the concentration of salts dissolved in the water

Many researchers have found that increases in conductivity (primarily caused by inputs of road salt) are strongly associated with urbanization of an area. When snow melts and washes road salts into rivers, it can cause a “toxic shock” to fish and other organisms. This toxicity may come not only from elevated chloride levels, but also from heavy metals, phosphorus, and anti-clumping agents in road salts. 

Two things could be happening with fish that would cause decreased prevalence of eggs in river systems with high influx of road salts. One possibility is that the eggs and larvae may die in response to sudden changes in salinity or other elements associated with road salts. The other possibility is that adult fish may become confused on their migrations if the chemistry of their natal stream is different than when they left it originally.

This study has been finding that herring spawning becomes more variable as development in a watershed increases. This may be due to not only the impact of road salts, but also changes in sedimentation or availability of suitable spawning habitat as development encroaches on a river or stream (other factors also examined by this study). The results of this study reflect the trends that many researchers are seeing along the Atlantic Coast of a relative increase in development concurrent with a relative decrease in herring populations. Studies such as this are important to demonstrate how common practices in our society (such as road salt application) may be influencing our rivers and streams, so that we may consider alternatives that will be friendlier to fish and other organisms that we value.