One of the puzzling developments in the marine system in the Northwest Atlantic recently has been the comparatively better success of the haddock stocks as compared to the cod. Both "groundfish," these two species have a lot in common - yet haddock are clearly faring better than cod these days. Explanations for this are very hard to come by, as described in this query from a local fisherman:

LETTER RECEIVED NOV. 27/2000:

Debbie,

I have watched the arguments on rebuilding of fish stocks or the lack of in some cases. In the 2j3kl cod, the stock didn't rebuild even after the fishery was closed. There was some older mature fish there but the spawning didn't seem to amount to anything.

I have a question that I have asked for a few years now without getting an answer. We have here in the Brown's Bank and Georges Bank area stocks of cod and haddock. These are found in the same area at the same time every year. The eggs of each species and also the larvae are found in the same area at the same time. Both act the same was such as the eggs are released in the water column and float to the surface where they hatch and after a while drift to the bottom.

The question I have is why do we have both species at the same place at the same time but we have seen haddock in record numbers the last 3 years but at the same time no cod recruit. We at the same time have around the same amount of spawning biomass. The 98 and 99 year classes of cod seem to be better than we have seem in the last 10 years. Why would one species be negatively impacted and not the other?

Claude d'Entremont

REPLY:

Claude,

Your question about the remarkable difference in the "recruitment" success of the cod and haddock stocks these days is a very good one. The two fish species have a lot of common characteristics, although cod have historically occupied a wider range. I read somewhere...can't remember exactly where now...that scientists used to assume that the two stocks would pretty much react to things in the same way since they are so similar - although as you point out, that is certainly not the case at present.

Cod and haddock, as you say, share similar spawning habits, and the movements and development of the early life stages are remarkably similar. Seasonal migration patterns are also similar (summer in shallower water, winter off in deeper water). One would also suspect that both cod and haddock would suffer the same effects from changing environmental conditions (which they do, both species grow a lot more slowly in colder water for instance). So the situation that you describe on Brown's and Georges Banks is very puzzling - but here is my answer:

One effect of the overall biomass depletion in the ecosystem is that sea life is now relatively more concentrated in the lower "trophic levels." This refers to how many steps up the food chain they are. It looks something like this: Phytoplankton => Zooplankton => filter feeders, small invertebrates => lower carnivorous fish => higher carnivorous fish (That would roughly represent trophic levels "1,2,3,4 and 5") Fish commonly move up the trophic levels as they age, the larval stages eat plankton, smaller fish eat little worms and other invertebrates, and when they grow to the bigger sizes fish eat smaller fish. Although you might not suspect it immediately, if you have adult specimens of cod and haddock that are the same size - say 20 inches long - they are not likely feeding at the same trophic level. Haddock is lower. The clue is in the size of the mouth, it is smaller in proportion to the size of the fish than is the mouth of the cod (scroll back up and look at the pictures again). That is because the haddock relies very heavily on feeding on the small invertebrates, and can subsist on them completely if need be - his mouth is actually too small for much else. The adult cod, on the other hand, is designed to be primarily a "fish" eater, his bigger mouth (plus analysis of stomach contents) reveals the fact that this fish was designed to live off larger prey...and "larger prey" are becoming a rather scarce commodity.

So the challenge of surviving by feeding at a lower trophic level is much tougher for the cod than it is for the haddock - and I think that provides the answer to your question. There has been a fair amount of comment in recent years describing the lowering of the trophic level of the fish that humans catch - one scientist, Dan Pauly, got quite a bit of press and (I think it was he who) coined the phrase "fishing down the web." This article gives a brief description of what he found:

http://naturalscience.com/ns/inter/inter01.html

Humans find themselves "fishing down the web" these days (heavy reliance on crustaceans, for instance, instead of finfish)...but they are not the only ones. Marine life in general is faced with the same challenge - essentially having to survive by eating littler stuff. The life-content of the system (total amount) is being gradually lowered, in effect diluted, and the system is now unable to support the higher trophic levels like it used to. Hence today's fishing success stories are all low-feeding species like lobster, scallop, shrimp, and some small flatfish and pelagics...while the fish that need to feed higher in the food web are increasingly in trouble (look at the very top ones, swordfish and bluefin tuna).

To show you what I mean about the significant differences in the diets of cod and haddock, here are a couple of quotes from a fisheries text ("Fishes of the Atlantic Coast of Canada" - Fisheries Research Board of Canada):

Re haddock: "The small subterminal mouth limits haddock to smaller articles of food, which consists predominantly of bottom forms including crustaceans, molluscs, echinoderms, annelids, and fishes. Over 200 species of invertebrates and fishes have been recognized from 15,000 haddock stomachs. BRITTLE-STARS AND BIVALVE MOLLUSCS MAKE UP THE BULK OF THE DIET but a complete list would include representatives of all the major groups of invertebrates. Haddock that are caught on the banks show a preference for fish as food and this consists almost entirely of the sand lance. Annelids constitute about 10% of the food of bank haddock" (page 204, "annelids" are worms)

Re cod: "Atlantic cod are voracious feeders. As fry they eat copepods, barnacle larvae, amphipods, and other small crustaceans. As young adults their chief food continues to be crustaceans such as shrimp, small lobsters, spider crabs, hermit crabs, euphausiids, and mysids. After cod reach a length of about 20 inches FISH BECOME THE PREDOMINANT FOOD, with herring ranking high; where available, capelin are eaten in quantity; on the offshore banks, sand lance are important in the diet; other fishes eaten include mackerel, redfish, hake, flounders, blennies, cunner, sculpins, silversides, shad, alewives, young cod, and young haddock. Molluscs form a less important part of the diet of Canadian cod." (page 198) (capitals mine)

A couple of other points come to mind - I recall reading that codfish is approximately twice as high in calories as haddock. It makes a richer meal for us. I presume that reflects a higher fat content, and probably implies that it takes more to "build" and sustain a codfish than a haddock. Also, haddock do not grow as big or gain weight as quickly as cod do. Where the stocks live together, a 5 year old cod, for instance, is noticeably larger than a 5 year old haddock. Adding it all together, I think the evidence points to the differing feeding requirements of mature cod and haddock as being the critical factor determining their relative success in today's marine environment.

So, Claude, that's my best guess.

Regarding your comment on the 2J3KL cod stock, the scientists don't report what you heard ("There was some older mature fish there but the spawning didn't seem to amount to anything.") It is actually rather contrary to that, since it's the older, mature cod that seem to have completely disappeared from the area. Some little cod still survive in "2J3KL", including a fair number of "0-group" (young of the year), but something has caused the disappearance of all the bigger, older ones. I think it's because that ecosystem has been lowered to the point that it will now only support the younger, lower-feeding members of the cod stock - and the trend is worsening over time. Surveys up till 1991 consistently reported cod up to age 12 years and older, between 1992 and 1994 there were very few fish older than 7 or 8, and from 1995 till present there have been very few cod found that were older than 5 years (this is getting scary since in that cod stock females mature at about 5 years - if the trend continues...the cod won't!) DFO's latest report on the 2J3KL cod stock is available online as a pdf file at: http://www.dfo-mpo.gc.ca/csas/csas/2000/a2-01e.pdf

(BTW, it also seems to me that the "cod vs haddock" conundrum argues against other explanations given for declining fish stocks these days: "pollution" should affect both fish, "overfishing" should not keep the cod down once the fishing is stopped, "habitat destruction" should affect both since they share the same area, and "excessive natural predation (like seals)"....why would seals have such a strong preference for cod? ...haddock tastes practically the same :)

All the best,
Debbie MacKenzie


(Reference: "Fishes of the Atlantic Coast of Canada" by A. H. Liem and W. B. Scott, Fisheries Research Board of Canada Bulletin No. 155, Ottawa 1966)

See also: update on Atlantic cod, posted Nov/2002