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How we develop our eco-ratings

To decide which fish merit an Eco-BestEco-OK or Eco-Worst rating, EDF scientists and our partners evaluate many aspects of wild fisheries and fish farming operations, and offer their analyses of more than 200 types of fish and shellfish common in the U.S. market.

Eco-Best choices represent fish whose fishing or farming methods have minor impacts on the environment, while Eco-Worst choices have considerable impacts on the environment. As fishing and farming practices change over time and new information becomes available, we re-evaluate our rankings.  Our guide is produced in collaboration with the Monterey Bay Aquarium, another producer of seafood eco-guides.

Below are some of the issues we consider for each species.

Wild fisheries

  • Life history - The biology of a species is one indicator for how it will respond to fishing pressure. Fish that grow quickly, reproduce at an early age, and have short life spans are more resilient and less likely to be overfished (e.g. mahimahi). Conversely, fish that reach sexual maturity at a late age and are long-lived are more susceptible to population decline as a result of fishing pressure (e.g. Pacific rockfish). Special behaviors, unique reproductive traits, restricted ranges and natural population variability are other factors we consider.
  • Abundance - Population size is another indicator of a fishery's status. If the population becomes too small due to overfishing or other problems, it can reach a point where it can't recover. Changes in population size, skewed age or sex ratios, and species with federal designation of "overfished," or internationally recognized designations such as "threatened" or "endangered," are additional factors that we consider.
  • Gear impacts on habitat - Types of fishing gear vary considerably in their impacts on habitat. Bottom trawling and dredging are examples of fishing methods that can damage the seafloor and habitat. Gear such as purse seines, handlines and pelagic longlines have little or no effect on fish habitat. Also considered are the length of time it may take a habitat to recover, protection of critical areas, fishing gear innovations, and non-fishery habitat impacts (e.g. pollution, coastal development).
  • Bycatch - Bycatch, or unintended catch, may be related to gear type or management strategies. For example, wild shrimp is often caught with large trawl nets dragged along the ocean floor that pull in bycatch often exceeding the intended catch. Other fisheries (e.g. stone crab) use very selective gear that pulls in little bycatch. Gear that often results in high bycatch levels are bottom trawls, pelagic longlines and dredges. Selective fishing methods include trolling, traps, and hook and line. Fishing gear modifications, government regulation, and bycatch of "overfished," "threatened" or "endangered" species are given additional consideration.
  • Management - Unregulated fisheries, or those with ineffective management, are susceptible to overfishing and ecosystem damage. Examples of fisheries with poor management include Chilean seabass, which is fished illegally over a large portion of its range. Another consideration is if there are too many boats. Signs of effective management include the ability to monitor fisheries and collect information, and innovative management solutions (e.g. catch shares and marine protected areas).

Below is a case study that demonstrates how we establish our rankings for wild-caught fisheries.

Case study #1 - Pacific halibut

  • Abundance - Pacific halibut populations are healthy.
  • Gear impacts - The predominant type of fishing gear used to catch Pacific halibut is bottom longlines. This type of gear has relatively little impact on bottom habitats in this fishery.
  • Bycatch - Use of bottom longline gear in this fishery results in low bycatch. In addition, bycatch is strictly regulated through a new management plan (see below), implemented at the request of fishermen in Alaska and Canada. Special methods developed by the fishing industry (e.g. fluorescent streamers and mandatory bait disposal techniques) minimize bycatch of albatrosses and other seabirds.
  • Management - The Pacific halibut fishery was converted from a 'derby' fishery to a catch share system in 1995. Under this innovative approach, U.S. and Canadian fishermen are allocated shares of the scientifically-set catch limit. They can harvest their shares any time during the year, and follow strict monitoring and enforcement rules. The result is that fishermen's financial incentives align with conservation objectives for the fishery. This new management system also results in safer working conditions, higher quality fish, better prices for fishermen, and longer fishing seasons.

Farmed Fish

  • System design - The environmental impacts of aquaculture (farmed) production are linked to the types of systems used to grow fish. For example, systems that are open to the environment (e.g. net pens) are usually associated with the greatest environmental harm (see case study below). Conversely, closed systems (e.g. recirculating tanks) do less harm. Since many species are raised by only one type of operation, system design is a good indicator for overall impact. Other factors that we consider are stocking density, siting practices, government policies and technological innovation.
  • Feed content - Some species of farm-raised fish are carnivorous and therefore require fishmeal and fish oil in their feed. To feed these fish, massive quantities of small, oily fish like anchovies, menhaden, mackerel and herring are caught to produce fishmeal and oil for feed. On the other hand, omnivorous and herbivorous fish like catfishtilapia and carp need relatively small amounts of fish products in their feed. Farmed mollusks (clamsmusselsoysters and scallops) require no feed at all, since they filter their food from the surrounding water. We also consider additional factors like feed conversion ratios (how efficiently a species converts feed to flesh), government policies and technological innovations in feed composition or efficiency.
  • Water pollution - Aquaculture operations vary in the amount of effluent (wastewater) they produce, and in the treatment of that effluent. Open systems like net pens produce large amounts of untreated waste and uneaten feed. These nutrients pass directly into surrounding waters, where they can significantly impact local ecosystems. Ponds (used to raise U.S. catfish and most shrimp) and recirculating systems (used for some finfish and shrimp production) are better able to control and treat effluents. Effluent treatment options such as settling ponds and constructed wetlands can be used to reduce water pollution from ponds and other types of fish farms. We also take into account government policies, technological innovations, and the discharge of chemicals (e.g. parasiticides, antibiotics).
  • Risk to other species - Farmed fish, if they escape, can pose serious threats to wild fish populations by competing with them for habitat, interbreeding and transmitting disease and parasites. Fish are most likely to escape from open systems, such as salmon net pens, since they are directly in surrounding waters. Farming native species, or raising fish in tanks, generally minimizes the possibility of escaped fish affecting native fish populations.
  • Ecosystem effects - The location of aquaculture facilities can determine their impact on surrounding ecosystems. Operations sited in areas of high ecological sensitivity (e.g. mangroves and coastal wetlands) have the greatest potential for harm (see case study below). Fish grown in land-based tank systems (e.g. a large percentage of hybrid striped bassU.S. tilapia and arctic char) or on converted agricultural land (e.g. catfish) are good examples of lower impact operations. Other factors that we consider are water use, habitat degradation, unnecessary predator deterrents (e.g. shootings or acoustic "scare" devices), collection practices to stock farms (e.g. depleting wild populations for "seed"), and government policies. Below is a case study that demonstrates how we establish our ranking for aquaculture species.

Case study #2 - Farmed salmon

  • Feed - Because salmon are carnivorous, typical salmon farming operations consume more fish than they produce. Globally, roughly three pounds of wild feeder fish are used to produce each pound of farmed salmon. Typical salmon farming therefore puts pressure on wild fish populations, rather than supplementing them.
  • Pollution - Typical salmon farming operations contribute to coastal water pollution. Fish wastes and uneaten feed can contribute to nutrient loading, which in some areas has led to algal blooms, or other water quality problems. The accumulation of wastes beneath net pens can significantly damage marine life on the sea floor.
  • Antibiotic/chemical use - A variety of chemicals are used in salmon farming, often including antibiotics, pesticides and parasiticides. These drugs are generally administered in feed, and can disperse into the environment, potentially affecting other organisms. In addition, the regular use of antibiotics on some salmon farms can contribute to the spread of antibiotic resistance in pathogens that infect fish and humans.
  • Escapes/disease - Salmon escape from farms in small numbers through "leakage" and also in large numbers episodically due to storms or other causes of damage to net pens. Depending on where farms are located, escaped fish may compete with native fish for habitat, or breed with them - in the latter case making the native fish population less genetically adapted to the local environment. Escaped farmed salmon may also spread parasites and pathogens to wild fish.
  • Predator control - Salmon farming operations sometimes harass or even kill wildlife such as seals or birds to discourage them from feeding on farmed salmon.
  • Environmental contaminants - Recent studies show that most farmed salmon contain significantly higher levels of PCBs, dioxins and pesticides than wild salmon. These contaminants appear to originate in salmon feed. Fishmeal and crude fish oil in feed can contain relatively high levels of contaminants, depending on their source.

Environmental Defense Fund works to reduce the discharge of contaminants, particularly mercury, which build up in fish. We also work to improve management of fisheries to help restore healthy fish populations and protect fish breeding habitat. These efforts will ultimately benefit the fishing industry as well as seafood consumers.