The question of “how much fish is in the ocean” is one of the most complex inquiries in marine biology, given the sheer scale and depth of the global ocean. While an exact count of individual organisms is impossible, scientists work to determine the total weight, or biomass, of all fish species. This answer is not a single, static number, but a dynamic estimate requiring continuous refinement based on advanced technology. Quantifying this immense biomass is fundamental to managing ocean health and understanding the global ecosystem.
Why Calculating the Total Is Nearly Impossible
Estimating the total weight of fish in the ocean is inherently difficult because the vast majority of the water column remains unexplored and inaccessible. The ocean covers over 70% of the Earth’s surface, creating a three-dimensional environment that is impossible to survey comprehensively. Most research efforts are limited to the surface waters and continental shelves, leaving the deep ocean largely unquantified.
This difficulty is best illustrated by the immense population living in the mesopelagic zone, often called the “twilight zone,” between 200 and 1,000 meters deep. Early estimates based on traditional net trawling suggested a modest biomass for these small, mid-water fish, such as lanternfish. Scientists later realized that these species are adept at detecting and evading nets, leading to a massive underestimation of their true numbers. Their ability to escape sampling gear meant a significant portion of the ocean’s total biomass was essentially invisible to conventional methods.
Methods Used to Estimate Marine Biomass
To overcome the limitations of physical sampling, scientists employ a combination of indirect and direct measurement techniques to estimate fish biomass. One widespread method is the acoustic survey, which uses ship-mounted echosounders to send sound waves into the water and measure the “backscatter,” or the intensity of the echoes bouncing back off organisms. This technique allows researchers to rapidly map the density and distribution of fish schools over large areas, particularly in the mid-water column.
Translating acoustic backscatter data into an accurate biomass figure is complex, requiring assumptions about the fish species present and their physical properties, such as the size of their swim bladders. This uncertainty means different acoustic surveys can yield different results. Traditional trawl surveys remain necessary for species identification and collecting biological data, despite their inefficiency for evasive species. For commercially exploited species, scientists use ecosystem modeling, which integrates historical catch data, stock assessments, and biological parameters to reconstruct population sizes over time.
Current Estimates of Global Fish Tonnage
The most significant change in understanding global fish tonnage came from a major revision of the mesopelagic population estimate. Previously, the biomass of fish in the twilight zone was estimated at around 1 billion tons. Acoustic data gathered during the Malaspina Expedition suggested this figure could be at least ten times higher, placing the mesopelagic biomass closer to 10 billion tons. This means the vast majority of the ocean’s total fish biomass is composed of small, non-commercial species inhabiting the deep, open ocean.
Commercially viable fish stocks—those targeted by fisheries—represent a much smaller, yet more directly tracked, fraction of the total. Estimates suggest that the pre-exploitation biomass of exploited fish, defined as those weighing between 10 grams and 100 kilograms, was approximately 3.3 ± 0.5 gigatonnes (Gt). Historical analysis shows that fishing practices have substantially reduced this biomass, with some models indicating a reduction of nearly half by the 1990s. While the total weight of all fish is dominated by billions of tons of small, deep-sea species, the biomass of fish humans consume is significantly lower and has been demonstrably reduced.
The Impact of Declining Fish Populations
A decline in the total weight of fish, particularly commercially exploited species, has consequences extending far beyond human food supply, primarily affecting the marine food web and the global carbon cycle. Fish serve as a fundamental link in the food chain, transferring energy from lower trophic levels to higher predators like seabirds and marine mammals. A reduction in fish stocks can trigger a trophic cascade, causing imbalances that affect the abundance of both their prey and their predators.
Fish play a significant role in the ocean’s biological pump, the process that sequesters carbon dioxide from the atmosphere into the deep sea. This occurs through mechanisms including the production of carbon-rich fecal pellets, which sink rapidly to the ocean floor. The vertical migration of many fish species, particularly those in the mesopelagic zone, also contributes to carbon transport by feeding in surface waters at night and migrating to deeper waters during the day. Studies suggest that the carbon sinking via fish excretion is substantial, contributing an estimated 1.5 to 1.65 billion tons of carbon annually, highlighting their importance in long-term carbon storage.