Schooling & Migration

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) Aggregation is not a shoal or school… How many fishes school? a group of fish formed by external factors, not social fish-fish attraction ~1600 species as juveniles ~4000 species as adults (2000 marine species) •
facultative schoolers (e.g., some parrotfishes) •
obligate schoolers (e.g., clupeids) 1 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 Degree of synchrony varies among species: 1) Arrangement •
all members doing same thing Quantification •
distance between individuals •
angles of orientation consider ratio of distance to 1st and 2nd schoolmate 1:1 would be perfect spacing Examples (from Partridge 1982) •
herring = 1 : 1.1 •
pollack = 1 : 1.3 •
cod = 1 : 1.5 - in perfect school, all distances & angles to nearest schoolmate identical - but not in reality... imperfect schooling in Pollack Partridge 1982 2)
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  increased distance between schoolmates •
Lateral line - no lateral line  decreased distance between schoolmates no vision or lateral line  no schooling 2 Is there a leader of the school? yes and no •
one fish does not set the exact speed and direction 3) Costs & Benefits of schooling •
presumably schooling has evolved because this behavior increases fitness •
maximize fitness by minimizing costs & maximizing benefits •
but, must be a fish at the front making decisions about which way to go Costs of schooling: Benefits of schooling: •
competition for food •
drafting •
transmission of disease & parasites •
increased feeding •
detection by predators •
mate location & reproductive synchronization •
decreased O2 in center •
improved migration •
increased waste products (ammonia) in center •
reduced risk of predation Drafting Increased Feeding Catalina Sea of Cortez jack mackerel grunts •
using vortices behind other fish to reduce swimming effort •
optimal spacing: 1/2 to 1 body length •
theoretically can reduce effort to swim by 65% Cooperative Hunting in yellowtail (Schmitt & Strand 1982) •
benefits unequal among individuals (none for fish at front) •
in reality, benefit is smaller because spacing is not optimal 3 Increased Feeding Mechanisms for reducing risk of predation in groups 1) Increased vigilance
improved food detection •
reduced time to detect food 2) Dilution effect •
increased detection distance 3) Learning Time to find food 4) Predator confusion Pitcher et al. 1982 2) Dilution effect 1) Increased vigilance •
able to detect predators earlier or farther away - spreading the risk •
if a predator takes one prey per encounter, and by yourself your risk is r Test in minnows & pike Magurran et al. 1985 Detection distance •
then in a group of size g, your risk will be r/g •
but… - predators often eat more than one prey item Group size 3) Learning lots Ferrari et al. 2005 4) Predator confusion •
inexperienced fish can learn to recognize predators from experienced group members - hard to focus on many moving prey Test in minnows & largemouth bass Landau & Terborgh 1986 Increasing predator odor  % trials ending in capture 1 - being in a group increases your risk of being found by a predator 100 90 80 70 60 50 40 30 20 10 0 0 5 10 # of minnows 15 4 Fountain Effect 4) Predator confusion responses of schools of prey to predators •
compaction •
fountain effect •
flash expansion Predators have developed hunting tactics that overcome the predator-confusion tactics Flash Expansion •
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) 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 more 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 5 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.. General types of migration in fishes a) more accurate homing of salmon: - averaging of individual error increases overall accuracy b) cultural transfer of information •
Oceanodromy -- entirely within marine waters •
Potadromy -- entirely within fresh water - e.g. grunts learn migration routes to feeding locations •
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 oceanodromous Migration Types anadromous Diadromy diadromous catadromous potadromous amphidromous 6 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 - 100’s of m - Myctophids: 300-1200 m deep in daytime, 10-100 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 white shark Oceanodromy 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 8 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