What Do Salmon Eat While at Sea?

“Of over 800 Chinook Salmon taken at sea off the Canterbury Coast, over 99 per cent of their diet comprised just 3 species.”

By Gavin James, NIWA, Christchurch

Anglers who fish for salmon in the sea, particularly in and around Otago Harbour, know that using sprats or pilchards as bait is often more successful than spinning using hardware. Over the last year or so, this method of fishing has spread to the lower reaches of some rivers, especially the Waimakariri and Kaiapoi, where anglers have had very good results using small fish as bait.

Now a publication describing the results of several years of work analysing the stomach contents of over 800 Chinook (or quinnat) salmon taken at sea off the Canterbury coast, has confirmed why small silvery fish make such good salmon bait and has also shed some light on where salmon feed when in the ocean.

Most of the stomachs examined were from 2 and 3-year-old salmon, taken as by-catch by trawlers fishing for red cod and barracouta, and were collected by verifiers present on the vessels as part of the Salmon By-catch Agreement between the Ministry of Fisheries and representatives of the commercial and recreational fishing sectors. We also examined a few 1-year-old salmon taken from inshore areas using other methods.

Trawler-caught salmon fed mainly on fish, with sprats making up 76 per cent of their diet and juvenile hoki 5 per cent of their diet. So anything that looks like a small silver-coloured fish – especially a sprat or a pilchard (or a sardine if you are only familiar with the tinned forms), is what salmon commonly feed on in the sea, and should be successful as bait. Salmon also ate the reddish swarming krill known as Munida, when these were present.

It is significant that sprat, swarming red krill, and juvenile hoki all live off the bottom (ie, are pelagic), and that virtually no bottom-dwelling prey were eaten by salmon. However, the conclusion that salmon are feeding on pelagic food seems inconsistent with the fact that these fish are caught by bottom trawlers. Since it is known that trawlers can increase the catch of salmon by increasing the height of the headline (unpublished Ministry of Fisheries report), it seems likely that salmon are living and feeding on pelagic prey within the lower 5 -10 m of the water column, but above the more demersal red cod that the trawlers often target.

Off the United States west coast, where chinook salmon are native, they are also taken incidentally by bottom trawlers, and their diet is composed almost entirely of pelagic organisms ­ both fish and krill, but different species to those in New Zealand.

Salmon in North America are generally regarded as opportunistic feeders, eating a wide range of marine organisms. So it was surprising that the fish we examined had fed on only a few different types of prey. Over 99 per cent of their diet comprised just 3 species: sprats, red krill, and juvenile hoki.

We know that New Zealand chinook salmon grow rapidly during their first 1- 2 years at sea when compared to North American stocks at similar latitudes, suggesting that growth rates are not generally limited by prey availability. However, the limited types of prey we recorded suggest that sprats and red krill are virtually the only suitable prey items present in the sampling area during summer, and a shortage of one or both of these short­lived and probably annually variable species could have a significant impact on growth and /or condition of adult chinook.

Sprat are abundant in the Canterbury Bight, with biomass (for both species combined) estimated to be at least 60,000 t, although there is no data on inter-annual variability. The abundance of pelagic red krill varies greatly from year to year, and a common belief among New Zealand salmon anglers is that the size and condition of salmon taken by anglers each autumn, are related to the abundance of krill during the preceding summer.

Angler-caught chinook from the Rakaia and Waitaki Rivers in 1994 were longer and heavier (by 25 per cent – 30 per cent) at age 3 years than those caught in 1992 and 1993 (NIWA, unpublished report), suggesting that fish in both rivers were subject to some common influence within the marine environment. We consider it likely this was the increased availability of red krill through January and February 1994, now recognised as a year with an extended red krill shoaling season.