Schooling & Migration

Transcription

1 Schooling & Migration Schooling in fishes What is a school? What makes a school a school? Definitions shoal -- a group of fish that stays together through social interactions Mixed Species: schools & shoals can include more than one species school -- a synchronized and polarized shoal (orientation & spacing non-random) 1

2 Aggregation is not a shoal or school a group of fish formed by external factors, not social fish-fish attraction How many fishes school? ~1600 species as juveniles ~4000 species as adults (2000 marine species) facultative schoolers (e.g., some parrotfishes) obligate schoolers (e.g., clupeids) Size of schools: Two (or three?) fish to hundreds of millions (e.g., 17 mile long schools of herrings have been seen) Aspects of schooling 1) arrangement 2) coordination 3) costs & benefits 1) Arrangement all members doing same thing Quantification distance between individuals angles of orientation - in perfect school, all distances & angles to nearest schoolmate identical - but not in reality... Degree of synchrony varies among species: consider ratio of distance to 1 st and 2 nd schoolmate 1:1 would be perfect spacing Examples (from Partridge 1982) herring = 1 : 1.1 pollack = 1 : 1.3 cod = 1 : 1.5 2

3 imperfect schooling in Pollack Partridge ) Coordination of school: how is arrangement maintained? vision brings fish together, lateral line repels them, the balance achieves proper spacing (Pitcher et al. 1976) saithe Vision - no vision è increased distance between schoolmates Lateral line - no lateral line è decreased distance between schoolmates no vision or lateral line no schooling Is there a leader of the school? yes and no one fish does not set the exact speed and direction but, must be a fish at the front making decisions about which way to go 3) Costs & Benefits of schooling presumably schooling has evolved because this behavior increases fitness maximize fitness by minimizing costs & maximizing benefits Costs of schooling: competition for food transmission of disease & parasites detection by predators decreased O 2 in center increased waste products (ammonia) in center Benefits of schooling: drafting increased feeding mate location & reproductive synchronization improved migration reduced risk of predation 3

4 Drafting using vortices behind other fish to reduce swimming effort optimal spacing: 1/2 to 1 body length Increased Feeding Cooperative Hunting in yellowtail Catalina jack mackerel Sea of Cortez grunts theoretically can reduce effort to swim by 65% (Schmitt & Strand 1982) benefits unequal among individuals (none for fish at front) in reality, benefit is smaller because spacing is not optimal Increased Feeding improved food detection reduced time to detect food increased detection distance Time to find food Mechanisms for reducing risk of predation in groups 1) Increased vigilance 2) Dilution effect 3) Learning 4) Predator confusion Pitcher et al ) Increased vigilance able to detect predators earlier or farther away Detection distance Test in minnows & pike Magurran et al Group size lots 2) Dilution effect - spreading the risk if a predator takes one prey per encounter, and by yourself your risk is r then in a group of size g, your risk will be r/g but - predators often eat more than one prey item - being in a group increases your risk of being found by a predator 4

5 3) Learning inexperienced fish can learn to recognize predators from experienced group members brook trout: predator fathead minnow: prey Ferrari et al Increasing predator odor è 4) Predator confusion - hard to focus on many moving prey Test in minnows & largemouth bass Landau & Terborgh 1986 % trials ending in capture # of minnows 4) Predator confusion Fountain Effect responses of schools of prey to predators compaction fountain effect flash expansion Flash Expansion Predators have developed hunting tactics that overcome the predator-confusion tactics killer whales slap their flukes to stun their prey billfishes slash their bills as they pass through schools to injure or kill the prey some species switch to ram feeding and swim through schools with their mouths open at high speed schooling predators may cooperate (e.g., yellowtail) 5

6 Where is the safest place to be in a school? Center? test by with silversides and black seabass (Parrish 1989) No: safest on periphery, center attacked most Summary: schooling fish should school (or shoal) when the fitness of group living exceeds the fitness of living alone many factors influence the costs and benefits of schooling BUT, don t be a straggler! stragglers were attacked most Migration Migration: mass movement from one habitat to another with characteristic regularity in time, or according to life history stage 25,000 mile migration by 200 lb bluefin tuna Can be: active or passive Purpose: feeding (e.g., grunts, Myctophids) breeding (e.g., Nassau grouper) wintering (seasonal movements) (e.g., tautog) combination (e.g., salmon) Migration and schools: Schooling behavior may aid in migrations.. more accurate homing of salmon: - averaging of individual error increases overall accuracy cultural transfer of information - e.g. grunts learn migration routes to feeding locations General types of migration in fishes Oceanodromy -- entirely within marine waters Potadromy -- entirely within fresh water Diadromy -- between fresh and marine waters Anadromy: most growth in marine, reproduction in fresh Catadromy: most growth in fresh, reproduction in marine Amphidromy: movement between fresh & marine for purposes other than spawning 6

7 oceanodromous Migration Types anadromous Diadromy diadromous catadromous potadromous amphidromous Scale of migration Which families are diadromous? -- primarily in primitive fishes, e.g., Lampreys (Petromyzontiformes) Sturgeons (Ascipenseriformes) Herrings (Clupeiformes) Salmonids (Salmoniformes) -- otherwise rare and intermittent for example, 11 Perciform families have species that are diadromous, but never the whole family 1. small fishes in streams & coral reef fishes: meters to 10 s of m 2. Diel vertical migrations: m s s of m - Myctophids: m deep in daytime, m deep at night 3. Oceanic migrations (oceanodromy): - Tuna: California to Japan, Florida to Norway - Bluefish: from Northeast USA to South America - Marlin: CA to Hawaii 4. Diadromous migrations: - Chinook salmon, 4000 km - Sturgeon, 1000 km - Lamprey, 100 km 7

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9 Oceanodromy white shark Examples: white sharks bluefin tuna albacore yellowtail South Africa to Australia and back (11,000 km; Bonfil et al. 2005) Farrallons to Hawaii & back Guadalupe Island to Hawaii & back yellowtail migrate from central Baja to So Cal in summer How do fish migrate? Cues Olfactory -- scent molecules; e.g., salmonids Visual -- landmarks; angle of sun (sun compass) Geomagnetic -- orient to earth s magnetic fields (compass); e.g., eels, hammerhead shark Temperature -- e.g., albacore follow 14 isotherm Salinity -- salmonids find freshwater outflow into ocean at mouths of rivers magnetite crystal Summary: migration fishes undertake migrations ranging widely in duration and distance travelled a variety of cues and senses are used to complete migrations migrations routes are sometimes learned, others instinctive 9