Adult orcas need to consume a mind boggling 193,211 to 286,331 calories per day to meet their requirements – a figure derived by measuring the needs of captive orcas (but because those whales are forced into an inactive couch-potato lifestyle, the scientists add adjustments for the considerably more active wild orcas). Onto this figure is added the estimated amount of indigestible matter present in fish. Given all the guesswork involved in determining the caloric requirement for an orca, these figures may seem overly precise – but scientists minimize the magnitude of error by not rounding the numbers off.
How many fish does it take to meet those needs? Well, this is where it gets sticky, because the calorie values vary among the different salmon species. Using this calorie calculator, I determined that if the orcas were eating only salmon fillets, it would take about 355 lbs of Chinook, 530 lbs of chum, 398 lbs of coho, 380 lb of sockeye, or 547 lbs of pink salmon daily to feed an active adult male orca. If the salmon were solid muscle, that translates to 14 (25 lb) Chinooks, 41 (12 lb) chums, 44 (9 lb) cohos, 54 (7 lb) sockeye, and a whopping 109 (5 lb) pink salmon.
However, orcas are able to eat salmon whole, and can digest most of the bones, fins etc, because they have a three-chambered stomach. The first chamber is a muscular widening at the base of the throat, and here the salmon is mashed and ground before it is passed to the acidic stomach. Shells and pebbles have been found in the first chamber and are believed to aid in grinding the fish. By time the orcas are done digesting there is not much matter left to excrete.
I have yet to determine how many calories are present in the eyes, bones and organs etc, but I checked a fish processing site and learned that a fish over 100 lbs will yield 50% of its body weight in fillets, while smaller fish only yield 30 to 40% by commercial methods.
Therefore, depending upon the calories available to the orcas in the 50% to 70% that we humans consider the waste by-product of fish, the actual number of salmon required for them will vary considerably. Assuming for a minute that the whole fish has the same number of calories as its muscle tissue, the math becomes pretty simple: 3.5 (100 lb) or 17.5 (20 lb) Chinooks are equivalent…until you factor in the increased hunting effort needed to catch all those smaller fish.
Another complication is that the nutritional status of salmon alters over time (particularly sodium, potassium, and lipid content) as the salmon undergo the changes needed for them to adapt to different conditions and meet their own dietary needs.
The minimum number of Chinook salmon, using all the optimized figures presented here is 17.5 per orca per day – young animals will require less, nursing females more. With the present number of southern resident orcas at 87, that is 1522.5 (20 lb) Chinooks daily, which is approximately 45,675 per month, or 548,100 per year.
To insure that the orcas actually catch what they need, two to more than three times the number of fish must be set aside, bringing the number to 1 million to 1.6 million for the bottom line. Using the accepted effort rate of 3.2, the number becomes about 1.75 million Chinook salmon – and this is assuming that all of the salmon are uniform in calories and nutrients, and that the whole fish contains the same calories as the flesh, minus the 15% to 20% already accounted for in the basic caloric needs.
It seems like a lot of fish, but by making a generous allowance for the orcas, more salmon will survive to reproduce, creating a greater abundance for us all.