Daniel Pauly* and Rainer Froese t *Fisheries Centre, University of British Columbia, and t International Centerfor Living Aquatic ResourcesManagement, Manila I. Major Adaptations of Fishes fish spawning usually suffer tremendousand largely II. Respiratory Constraints to Growth and Related unpredictable mortalities, thus uncoupling spawn- Processes ing from recruitment. III. Distribution of Exploited Fish Stocks trophic level A number indicating the position of a IV. Ecosystem Impacts of Fisheries specieswithin an ecosystemthough the number of V. Managing Fish Biodiversity Information stepslinking it to the plants. By definition, plants VI. Preserving Fish Biodiversity are TL = 1, herbivoresare TL = 2, and so on. Note that trophic levelsdo not needto be whole numbers; intermediate values occur among omnivorous con- GLOSSARY sumers. biomass Collective weight or massof all the members of a given population or stock at a given time, or, on the average,over a certain time period. FISH STOCKSARE POPULATIONS OF "FISH," THAT bioquads Occurrencerecord of organisms,serving as IS, VERTEBRATESWITH GILLS AND FINS, SUB- key units for biodiversity researchand consistingof jECTED TO EXPLOITATIONBY HUMANS. Popula- four elements(species names, location,. time, and tions are componentsof species,inhabiting part of their source). overall range,and usuallyhaving little geneticexchange catches The fish (or otheraquatic organisms)ora given with adjacentpopulations. The major adaptations de- stock killed during a certainperiod by the operation termining the spatial distribution of fish stock biomass of fishing gear(s). This definition implies that fish pertainto the anatomy,reproductive biology, and respi- not landed, that is, discardedat sea,or killed by lost ratory physiology of the speciesto which the stocks gear ("ghost fishing"), should be counted as pan of belong.Also, fishing hasbecome increasingly important the catch of.a fishery. to the biodiversity of fish, either through its direct im- ecosystem Area where a setof speciesinteract in char- pacts (changes of stock size and age structure, and acteristic fashion,and generateamong thembiomass overallbiomass reductions, down to extirpation of pop- flows that are stronger than those linking that area ulations), or by modifying the ecosystemsin which to adjacentones. they are embedded. Researchdevoted to monitoring recruitment Entry of juvenile fish into the (adult) the biodiversity of fish (or other organisms) must be stock. Recruitmentis distinguished from reproduc- able to handle large amountsof suitably formatted dis- tion, becausethe eggsand larvae that result from tributional information, here defined as consisting of Encyclopedia of Biodiversity, Volume 2 Copyright @ 2001 by Academic Press. All rights of reproduction in any form reserved. 801 802 FISH STOCKS "bioquads." Managementregimes aiming at preserving breathers,to perceivethe constraintsunder which fish, fish biodiversity will haveto include much stricter regu- as water breathers,were forced to evolve. lation of fishing and the establishmentof no-takeareas. Water is an extremely dense medium, 775 times heavierand 55 times more viscous than air. Also, water contains30 times less oxygenthan air, and this oxygen I. MAjOR ADAPTATIONSOF FISHES diffuses 300,000 times more slowly than in air. These physical constraints,which shapedall early life-forms, A. Anatomyand Physiology including the jawlesspredecessors of the fish, the agna- thans,are bestvisualized by describingthe major evolu- With about 25,000 recognized species in over 500 fami- tionary trends leading from agnathansto modem fish lies, fish are the most diverse vertebrate group. How- (Fig. lA). ever, their watery habitat, while failing to protect them The first of these trends was the evolution of jaws from modem fishing gear, makes it difficult to fully from the first upper and lower gill archesof agnathans. appreciate this diversity, and the extent to which it is This built on the intimate connection,in the mostprim- now threatened. It is even more difficult for us, as air itive vertebrates,between the feeding apparatus (i.e., Thunnus obesus, A = 7.5 Leiognathus dussumieri t Pterolepis nitidus (15 cm) Cyprinus carpio, A = 2.2 Gazzaminuta Pomatochistus minutus, A = 0.6 Secutor ruconius Cyprinus carpio (1m) Cetorhinus maximus (15m) FIGURE1 Major evolutionary trends from agnathansto extant fishes. (Note that no direct ancestor-descendantrelationships are implied among the groups depicted.) (A) Trends toward larger gills; (B) trends toward efficient jaws; (C) trends toward effective paired and unpaired fins. [Note the aspectratio of the caudal fin, defined by A = h2/s,where h is the height and s the surface (in black) of the caudal fin, and of which high values define fast, large-gilled continuous swimmers, and converselyfor low values.) . FISH STOCKS 803 the mouth) and the respiratoryapparatus (i.e., the gills adjacentto slits on both sides of the anterior part of the alimentary canal). Water-breathing invertebrates lack this close connectionbetween feeding and breath- ing, one reasonwhy eventhe largestamong them (giant squids) cannot reach the mass of the largest fish (20 metric tons, for the whale shark Rhincodontypus). The reorganizationof the head of early fish allowed larger gills to evolve, which allowed the higher meta- bolic ratesrequired for swimming in openwaters. This transition was assistedby the gradualloss of the heavy armor protecting the slow, bottom-slurping agnathans. The fine "teeth" covering the bodies of sharksare ves- tiges of this armor. Fast swimming in open water required better fins, both for propulsion and for steering.Propulsion is pro- vided in most fish by oscillationsof a caudalfin whose aspectratio (Fig. lC) graduallyincreased toward tunas and other derived, fast-swimming groups with very large gills. Steering,on the other hand, is provided by dorsal, pectoral, and anal fins. These fins are stiffened FIGURE2 Schematicrepresentation of how changes in water level for preciseaction by hard, bony rays in the mostderived canmultiply, by creatingisolated subpopulations, the numberof species fish, the teleosts,whose evolutionary successwas fur- occurring in a givenarea. Such a mechanism,driven by repeatedclimatic ther enhancedby a complexly built protrusile mouth changes,is thought to explain the large number of fish speciesin SoutheastAsian marine waters and in the Great Lakes of Africa. that enables capture of a wide range of food items (Fig. 1B). Subtle anatomical changesin fish can thus create more niches for increasingthe numbers of specialists, larvae encounter, even during spawning seasonsat- which then occupy increasing numbers of closely tuned with zooplankton production cycles,are usually packedniches. Ecosystemsin which thesechanges have far too low to allow survival of fish larvae, and the run for long periods,undisturbed by physical changes, overwhelming majority of such larvae perish. Those therefore contain very large numbers of fish species. that tend to survive usually happenedto have hatched Their numbersare evenlarger in areassuch as the Great within plankton-rich water layers.These layers are usu- Lakes of Africa and the tropical Indo-Pacific, where ally a few centimetersthick and last for only a few days changesof water levelshave repeatedlyisolated basins of calm, betweenwind-driven or other mixing events, and subpopulations,thereby accelerating species differ- suchas storms or upwelling pulses,that enrich surface entiation (Fig. 2). waters with nutrients from deeperwaters. This implies that large biomassesof fish can build up only when and where the local oceanographicconditions take the form of "triads" defined by (1) nutrient enrichment, B. Reproduction and Recruitment such as generated by wind-driven mixing, (2) high Though many ancient fishes such as sharks and rays plankton concentration, such as generatedby various or the coelacanth Latimeria chalumnaepractice internal mechanismsincluding fronts, and (3) retention of lar- fertilization and produce few large eggs or live offspring, vae, required to prevent these weak swimmers from most recently evolved fishes produce numerous small drifting away from suitable habitat. In pelagic fishes eggs that are fertilized externally and develop as part that build high biomass, for example, the anchovies of the plankton, without parental care. The larvae that andsardines in coastalupwelling systemsoff northwest- emerge from those eggs, after less than one day in warm em and southwesternAfrica, Peru,and California,these tropical waters and up to two weeks (and more for larger triads occur only when the coastalwinds rangefrom 4 to eggs) in cold temperate waters, are usually elongated, as 6 m per second.Weaker winds do not generateenough befit small, finless zooplankton feeders. enrichment,and strongerwinds dispersethe larvae off- The average zooplankton concentrations that these shore. 804 FISH STOCKS Fish have developed several strategies to deal with declines with size, becausethe two-dimensional gill the uncertain recruitment that results from the triad area cannot keep up with the three-dimensionalin- requirements. One is being small, shott-lived, and capa- creaseof body mass.Hence larger fish disposeof rela- ble of quickly building up large biomass under favorable tively less oxygen to supply their metabolism,the rea- environmental conditions. The other is being large, son why they ultimately stop growing. Also, long-lived, and capable of weathering long series of environmental