On our page on the effects of diet choices on animals, we use statistics regarding the numbers of animals raised and killed each year to be eaten by humans. While there are good sources for these statistics when it comes to land animals in the U.S., the situation for fish and shellfish is significantly more complicated. While we also consider the numbers estimated by other sources, here we present our own calculations, which we refer to on that page.
We would like to determine the total number of fish that are killed, on average, on a per-capita level, to meet the demand within the U.S. This includes (a) the fish and shellfish that are killed and consumed directly and (b) the fish that are reduced to fishmeal and fed to farmed fish in aquaculture. In the first case, we have two further subdivisions: fish and shellfish that are wild-caught and those that are farmed.
Ideally, we would like to have the complete data for each category and then we can sum these numbers in a straightforward way, and calculate the final value. Unfortunately, the data provided by NOAA Marine Fisheries does not break down the numbers down to these categories in a uniform way. Moreover, most of the quantities in the tables and charts are expressed in terms of fish mass and not by their actual numbers.
With these constraints, we will first estimate the number of fish and shellfish in the final supply. We will then estimate the proportions produced in aquaculture. Next, we will estimate the amount of wild-caught fish needed to produce 1 pound of farmed fish. Using these findings, we will estimate the number of wild-caught fish that are used as feed in aquaculture. Finally, we will bring everything together to estimate the number of fish and shellfish used yearly by the average American.
Total number of fish and shellfish consumed directly
The NOAA Fisheries statistics states that the total amount of edible finfish () (excluding shellfish) by live weight in U.S. supply is 3.6 million metric tons (We are going to be consistently using metric units here).
The challenge is to convert this figure into the total numbers of fish. Our methodology is to consider a few dominant species of fish in the U.S. supply, and using their average live weight, calculate their respective numbers. We then use the average weight of fish thus determined to calculate the numbers of the rest of the species.
|Species||Type||Total Weight in U.S. Supply†||Equivalent Live Weight*†||Average Weight of a Fish (kg)||Number of Fish (millions)|
|Tuna||Fresh and Frozen||557||557||4||140|
|Salmon||Fresh and Frozen||519||519||2.81||184|
*Where the total weight is not provided as live weight, we use conversion ratios. For canned salmon and tuna, the conversion between the canned weight and live weight is 1:2.5.
**These are the numbers calculated for steaks and fillets made from groundfish. Look at table T1 in the Appendix for details of these calculations.
†Units are in thousand metric tons.
In the above table we have accounted for 2.3 of the 3.6 million metric tons of fish in U.S. supply. Under the assumption that the ratio of the total number of fish to the weight holds for the remaining 1.3 million metric tons of fish, we get that the total number of fish in U.S. supply is 1.3-2.34 billion.
Equivalently, on a per-capita basis, the number of fish consumed directly is 4.14 – 7.45. (Note that this includes both wild-caught and factory farmed fish.)
Turning to shellfish numbers, here is a table with the details of the main species in the U.S. supply.
|Species||Total supply†||Mean weight (kg)||Estimate of total numbers. (billion)||Per capita|
†Units are in million metric tons.
The above nearly accounts for all of the supply of shellfish, at least in terms of weight.
From the table above, about 186 shellfish per capita are consumed yearly, with the vast majority of these (160) being shrimp.
Proportion of fish that is farmed
The U.S. imports about 91% of its seafood, and half of that comes from aquaculture. We assume that this ratio applies to the entire U.S. supply of fish, including those caught or farmed domestically.
In other words 50% () of all the fish consumed in the U.S is farmed. This translates to about 2.07-3.73 farmed fish per individual in the U.S.
The proportion of shellfish that comes from aquaculture varies widely by species. Shrimp is the dominant species, both by weight and by numbers, and is subject to special conditions because of regulations on the import of wild-caught shrimp. About 93% of the U.S. supply of shrimp is imported, and more than 80% of that is from aquaculture. Most of the remaining 7% is wild-caught, so in total about 74% of the supply of shrimp is from aquaculture, or about 118 shrimp per capita per year. Crab and lobster are typically not farmed. Mollusks, including oysters, clams, and scallops, are regularly farmed, but farming methods more closely resemble conditions in the wild than for finfish and shrimp aquaculture, and fishmeal is not needed as a food source.
We want to calculate the quantity of wild-caught fish that is used as feed for the fish in the U.S. supply that is a product of aquaculture. While the crucial Fish-In-Fish-Out (FIFO) ratio varies depending on the species and also the location, we are going to use global averages in this case. Part of the justification for this approach is that almost all of the aquaculture that provides the fish consumed in the U.S. is situated outside the country, and most of it is concentrated in Asia. This region also happens to be the most dominant in the world for aquaculture. Thus global statistics on the amount of fish that is turned into feed would be a good estimate for the aquaculture systems in Asia, and thus the farmed fish that enters U.S. supply. It should also be noted that there have been some errors in calculating FIFO ratios, often with the result of overestimating their values for species like salmon.
According to the FAO report, the global aquaculture production (including fish and shellfish) in 2011 stood at nearly 54 million metric tons, and 15 million tons of (presumably wild-caught) fish were reduced to fishmeal and oil. About 70% of that, 10.5 million tons, is used in aquaculture. While there is no statistic on what fraction of this feed originated from finfish, various sources claim that the majority of fishmeal is produced from finfish. Additionally, while fishmeal is produced from different species in different parts of the world, in every country that is a major producer of fishmeal, a finfish species plays the major role in the country’s fishmeal industry. Therefore, we will assume for simplicity that fishmeal is produced entirely from finfish. This gives a figure of 10.5 million tons, corresponding to a FIFO ratio very close to 1:5. (It is to be noted that this method of calculation gives lower values—i.e., more efficient systems—for the FIFO whereas other sources—including those that represent the industries—claim ratios that are higher.)
Number of wild-caught fish used as feed in aquaculture
To calculate the number of fish reduced to fishmeal that is fed to the farmed fish which are consumed in the U.S., we not only need the FIFO ratio but also the average weight of a fish that is processed into fishmeal.
Using the data table provided by Fishcount, which is based on the FAO statistics and tabulates the various species of fish turned to fishmeal/fish oil, we estimate the average live weight of fish reduced to fishmeal is 31 gm, with an error of 8 gm.
Thus, we have
: Number of fish reduced to fishmeal () to feed farmed finfish () consumed in the U.S.
: Total weight of edible finfish () in the U.S. supply.
: Proportion of edible fish in the US supply that are farmed
Our analysis of finfish numbers is incomplete without considering the fish that are reduced to fishmeal and used in the cultivation of shellfish. Of all the shellfish consumed in the U.S., the most dominant in total weight and numbers is shrimp (clams and oysters and also farmed, and are significant in total weight, but they feed on planktons and so can be ignored in this analysis; almost all lobsters are wild-caught and that is mostly true with crabs as well). About 74 percent of all shrimp in U.S. supply is a product of aquaculture.
Again, we assume the same FIFO ratio of 1:5 and average weight of fish reduced to fishmeal to be 31 gms, and we obtain:
Adding both numbers of fish reduced to fishmeal gives us an estimate for the number of finfish reduced to feed for farmed fish and shellfish consumed in the U.S:
This translates to 57 fish per person. The corresponding error due to the uncertainty in the average weight of the fish is 15, giving an approximate range of 42-72 fish per person.
Putting it all together, on a per capita basis, we have:
|total fish consumed directly||4.14-7.45|
|total shellfish consumed directly||186|
|fish reduced to fishmeal/ fish oil for the farmed fish and shellfish entering the US supply||42-72|
|total fish accounted for||46-79|
|total fish and shellfish accounted for||232-265|
The total numbers of fish and shellfish affected by the average American diet are quite high, particularly when the use of small wild-caught fish in producing fishmeal and oil is accounted for. Although our calculations cannot be taken as exact due to the nature of the data collected by government agencies, which does not account for individual fish or shellfish, the magnitudes of the numbers are striking.
|Species||Round Weight of Fish Species.|
|Total U.S. Supply by Weight|
|U.S. Supply by Number of Fish|
☨ CF: Conversion Factor: It is taken to be 2.45 for all species based on FAO information.
*Units are in metric tons.