Estuaries and Larval Transport

1
Estuaries and Larval Transport

• Spawning and estuary rearing
• Processes of Larval migration/retention
• Crustaceans
• Seasonal spawning and transport
• American eels something really different!

Chesapeake Bay
2
Variation in abundance characterizes many
populations, and we often attribute them to human
actions (fishing, pollution, etc.)
But what about the pre-historic period?
Pacific sardine
Estimated Biomass (m mt)
Northern anchovy
1900
3
Variation in abundance characterizes many
populations, and these fluctuations often result
from entirely natural processes
Estimated Biomass (m mt) based on scales
deposited in Santa Barbara basin sediment
4
Classic spawner recruit relationship
0 2 4 6 8 10 12
Recruits in billions
0 1 2 3 4 5 6 7 8
9 10 11 12 13 14 15 16 17
Spawners in billions
Eastern Bering Sea walleye pollock
spawner-recruit relationship, 1964 -1988
year-classes.
5
Great variation characterizes many patterns.
There is some measurement error but a lot of true
variation.
0 2 4 6 8 10 12
Recruits in billions
0 1 2 3 4 5 6 7 8
9 10 11 12 13 14 15 16 17
Spawners in billions
Eastern Bering Sea walleye pollock
spawner-recruit relationship, 1964 -1988
year-classes
6
Age Class Phenomenon
Age distribution of North Sea Herring catch
Percentage of sample
Age of herring (years)
7
Year class strength and population variation

• If the species is long-lived and iteroparous the
  virgin biomass may be stable despite variation in
  recruitment success
• If the species is short-lived and semelparous or
  nearly so, the biomass may fluctuate widely and
  relative abundance of species may also shift

8
Relationship between size of eggs (E), yolk sac
larvae (Y) and fry (F) and mortality rate. Each
point is an estimate from a different study
lines indicate ranges.
Mortality ( per day)
(Bailey and Houde 1989)
Length (mm)
9
Why is there so much variation in recruitment
from year to year? Match-mismatch hypothesis
Abundance
Time
Cushing (1990) Advances in Marine Biology
26250-294
10

• There is a tide in the affairs of men which,
  taken at the flood, leads on to fortune omitted,
  all the voyage of their life is bound in shallows
  and in miseries. On such a full sea are we now
  afloat and we must take the current when it
  serves, or lose our ventures.

11
Examples of migration and estuarine occupancy by
Atlantic fishes of North America. Note the
diversity of spawning locations and patterns of
larval transport and adult migration.
Alewife, blueback herring, American shad
Menhaden
12
Generalized Penaeid shrimp life cycle
Estuaries
Inner continental shelf
13
Life Cycle of the Blue Crab (Callinectes sapidus)
Ocean
Estuary
14
Measurement techniques
Buoy
Flow meter
Zoeae or megalopae
Plankton net
Seabed
15
Tidal and diel movement of Uca spp.
Zoeae out to sea on the ebb tide NO DIEL PATTERN
Spring
Neap
Day
Night
Larvae per square meter
0 1 2 3 4
night
Ebb tide
12 0 12 0 0 12 0 12 0

Time of Day
Flood tide
Little and Epifanio 1990
16
Tidal and diel movement of Uca spp.
Megalopae return to estuary on flood tide. DIEL
PATTERN movement at night
Spring
Neap
Day
Night
Larvae per square meter
0 10 20 30 40 50
night
Ebb tide
12 0 12 0 0 12 0 12 0
Time of Day
Flood tide
Littel and Epifanio 1990
17
Blue crab (Callinectes sapidus)
Consecutive dates
Consecutive dates
Average abundance of megalopae collected weekly
during consecutive ebb and flood tides from
August 5 to October 28, 1986 (Little and Epifanio
1991).
18
Fiddler Crab (Uca spp.)
Ebb tide
Flood tide
1 5 10
Megalopae per square meter
0 10 20 30 40 50
Consecutive dates
Average abundance of megalopae collected weekly
during consecutive ebb and flood tides from
August 5 to October 28, 1986 (Little and Epifanio
1991).
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Retention of larval crabs in an estuary three
options
A. Larvae maintain a deep position throughout
development and are transported upstream by
non-tidal flow
Passive transport
bottom
B. Larvae sink in water column as development
proceeds.
Early stage larvae
Late stage larvae
bottom
C. Larvae migrate rhythmically with tidal flow
Epifanio 1988
20
Cross-shelf transport of estuarine larvae
A. Larvae drift offshore in surface flow and
swim back to the estuary
B. Larvae drift offshore in surface flow and
sink to deeper depths as development proceeds
C. Larvae drift offshore in surface flow and
return by wave transport
D. Larvae are transported away from estuary by
surface plume and transported back to vicinity of
estuary by wind-driven transport.
21
Seasonal spawning and transport

• North Carolina 7 fishes make up 92 of the
  commercial catch. All are estuary-dependent.
• Croaker (Micropogonids undulatus)
• Spot (Leiostomus xanthurus)
• Flounder (Paralichthys spp.)
• Menhaden (Brevoortia tyrannus)
• These four are late fall-winter spawners in
  continental shelf waters. Juveniles rear in
  estuaries in winter and spring. Sub-adult leave
  estuaries in fall.

So, why spawn in winter?
Atlantic Croaker
22
Why spawn in winter?

• H1 Low water temp Long survival at low
  rations
• H2 Reduced Predation
• a) Low predator ration
• b) Predators move offshore or south
• H3 Winter conditions favor on-shore transport

Spot
Flounder
Menhaden
23
Ekman flow
Water velocity decreases and rotates in direction
with increasing depth The water does not go in
the same direction as the wind.
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Why spawn in winter?

• North Carolina wind and water movement (Miller et
  al. 1984)

5-15 m
10-20 cm/s
2-5 m
5-15 cm/s
5-15 m
10-20 cm/s
25
Migration from spawning grounds in the tropical
western Atlantic to freshwater habitats by the
American eel (Anguilla rostrata) and European eel
(A. anguilla)
Larval Migration
Spawning area
McDowall 1988
26
American eel (Anguilla rostrata)

• Spawn in Sargasso Sea Feb-June
• Fecundity 15-20 million eggs
• Yolk sac stage a few days
• Leptocephalus stage 1-2 years
• A. rostrata distributed by coastal current from
  northern South America to Greenland
• A. anguilla distributed from Scandinavia to the
  Mediterranean Sea
• Glass eels ascend rivers via selective tidal
  stream transport in spring

Leptocephalus eel
Glass eel
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American eel (Anguilla rostrata)

• Females
• Found throughout the range
• Mature 60-90 cm, 8-14 years old
• Rear inland, grow slowly
• Males
• Predominate at the southern end of the range
• Mature 30-40 cm, 5 years old
• Fast growth, live near the coast
• Panmictic Population Structure
• Generalists Rivers do not contain discrete
  populations because juveniles do not rear in the
  river where their parents reared