History of Salmon and Relevant Literature

INTRODUCTION TO THE HISTORY OF SALMON CONSERVATION

If we are to conserve Salmon, it is important to understand the relationship that Salmon have with streams. Current conservation efforts usually focus on the cyclical nature of Salmon and Marine Derived Nutrients (MDN), mostly focusing on the relationship between Nitrogen and Phosphorus. The basic idea is this:

• Salmon deposit carcasses all along the freshwater streams in which they spawn, bringing with them nutrients gained at sea. 
• These nutrients fuel primary production (algae, other plant matter) which make up the base of the food chain in aquatic systems.
• This deposit of MDN allows for a healthy ecosystem, which in turn allows salmon alevine and fry (baby salmon) to grow. 
• The Cyclical Decline Hypothesis: Removing Salmon --> Removes MDN --> Lowers Salmon offspring survival --> Lowering Salmon returns--> Lowering MDN --> Lowering salmon offspring survival etc. etc. etc. 

'The Cyclical Decline Hypothesis' took quite an effort by biologists and ecologists to formulate and test (and it is still under debate!). Below I discuss how this proposed cycle came to be.

HISTORY OF SALMON AND RELEVANT LITERATURE

Primary production in streams is a function of several factors, but two of the most important are the ratio of Nitrogen to Phosphorous (N-P), and Carbon uptake/incorporation . This relationship has been known to affect primary production since the mid to late 1800’s. Methods of determining exact ratios of N-P nutrient ratios and thus primary productivity in aquatic environments were developed in the 1920’s (Goldman, 1960).  However, it was not until the mid 1900’s that scientists started studying exact relationships of N-P and salmon runs. One such study, done in the early 1970s, compared N-P ratios in sub-alpine streams in Alaska during high Kokanee Salmon (Oncorhynchus nerka) spawner abundance and low spawner abundance in two consecutive years(Richey, et al.,  1975). This study took samples of periphyton biomass, nutrient concentration, and heterotrophic activity to deduce Phosphorous levels during climax carcass deposits for both years. Their findings showed a positive downstream correlation with phosphorous and carcass deposit number, while low spawner seasons had a homogeneous nutrient spectrum at both upstream and downstream locations. In addition, bacterial heterotrophy and periphyton biomass were at maximum levels in midwinter (during peak salmon carcass deposit), an unusual time for an increase in primary production in Alaska. The authors conclude their study by stating that Kokanee salmon nutrient deposits via carcass vectors are important in preserving the rearing grounds of their own species.

A school of Kokanee Salmon returning to their spawning grounds. 

                This proposed cyclical relationship, where salmon carcasses create an influx of nutrients thereby generating more fertile spawning grounds, is suggested in subsequent research also. Stable isotope measurements have become a standard way of measuring nutrient uptake in streams, most commonly using Nitrogen (¹⁵N) and Carbon (¹³C).  A 1996 study of ¹⁵N and ¹³C isotope uptake in streams supporting Coho salmon (Oncorhynchus kisutch) runs bolsters the hypothesis that salmon carcass deposits increase primary productivity and act as net importers of marine derived nutrients (MDN) (Bilby, et al., 1996).  Salmon derived C and N were found to contribute heavily to the chemical make-up of organisms living in these systems; 44.8% N in cutthroat trout (Oncorhynchus clarkii) was marine derived, and 44% marine derived C was found in Coho salmon smolt. This suggests that incorporation of nutrients is due primarily to ingestion in organic form (prior to mineralization); primary production may not be a large sink of MDN from salmon. The concept, however, is the same; salmon are net importers of nutrients to primarily oligotrophic systems. Bilby concludes his 1996 article by stating that salmon declines may be self-perpetuating because the loss of MDN affects future generations of salmon runs.

This is a picture of a salmon carcass dump (via helicopter), which has been used in conservation efforts to replace MDN to systems with decreased salmon runs. This approach may not be the best conservation method, new papers suggest. 

                Further support for this hypothesis comes from Ted Gresch’s 2000 paper, which estimates salmon abundance throughout history (Gresch, et al., 2000). This influential article took data from cannery records throughout the late 1800’s and current harvest data to estimate the size of salmon decline along the west coast of North America.  Gresch reports that just 6-7% of historic salmon numbers remain, down from 160-226 million kg to 11.8-13.7 million kg.  He uses his data to extrapolate the hypothesis that in areas where salmon declines have been great there is a correlation of nutrient deficiency. Furthermore, he states, that “…current management,  which maximizes wild  salmon harvest,  relying heavily on artificial propagation may be exacerbating  the removal  of valuable  nutrients and carbon  from the  natural stream ecosystems,  perpetuating the low survival of wild salmon.”

Future Salmon!

 This cyclical decline hypothesis, prominent since 1970’s, has influenced the methodology of many conservation efforts.  The thought has been that since salmon appear to be great sources of nutrients to their oligotrophic spawning ground systems, addition of nutrients to said systems will increase salmon recruitment.  Restoration projects in the Pacific North West have added nutrients to rivers using several different methods: salmon carcass placement, ‘analog carcasses’ (carcass cakes composed of oceanic fish), and inorganic nutrient addition (Compton, et al. 2006).  These supplementations may negatively affect water quality, increasing chances of introduced toxins and pathogens.  Salmon population numbers have been slow to recover, even with carcass supplementation. Furthermore, recent evidence suggests that nutrient additions may not necessarily benefit salmon recruitment as much as once thought. If salmon carcass deposit is an ineffective and unnecessarily exhaustive process, conservation techniques may need an overhaul.