Realized connectivity: Larval source affectsRealized connectivity: Larval source affectspostsettlement survivalpostsettlement survival

Scott Hamilton, Bob WarnerScott Hamilton, Bob Warner

The importance to recruitment of some retention of local The importance to recruitment of some retention of local production has been stressed in several papers:production has been stressed in several papers:

“ “Up to 60% of recruits come from local production”Up to 60% of recruits come from local production”

““Large recruitment pulses tend to be retention events”Large recruitment pulses tend to be retention events”

- but these are based on numbers at settlement- but these are based on numbers at settlement

How do we know larval history?How do we know larval history?Otoliths and statoliths incorporate trace elements and Otoliths and statoliths incorporate trace elements and

can be used as environmental recorders can be used as environmental recorders

Dissolved trace Dissolved trace elementselements

Larval Larval uptakeuptake

Otolith Otolith incorporationincorporation

CORE HAS A NATAL SIGNATURE

SUCCESSIVE SUCCESSIVE LAYERS HAVE LAYERS HAVE INFORMATION INFORMATION ON LARVAL ON LARVAL PATHWAYSPATHWAYS

Core (Embryonic

period)

Early larval period

Near time of capture

Otoliths and statoliths also record age, growth, and Otoliths and statoliths also record age, growth, and condition at settlement condition at settlement

Waters near islands tend to be more productive and Waters near islands tend to be more productive and are enriched in many trace elements are enriched in many trace elements

Larval developmental period

Hatch Beginning Early Middle Late End

Pb

/Ca

( m

ol m

ol-1

)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

High Pb water massLow Pb water mass

Maximum concentration recorded in oceanic larvae

Recruits to St. Croix fall into two groups: high Pb over the entire larval life (nearshore) and low Pb in middle part of larval life (offshore)

Metamorphic band width (an otolith character) correlates strongly with condition at settlement

Lipi

d co

nten

t

(mg

g-1)

100

200

300

400

Met

amor

phic

ban

d w

idth

( m

)

10

20

30

PLD

(d)

40

50

60

Oto

lith

radi

us a

t se

ttle

men

t (

m)

160

200

240

A

C D

E

High Pb (nearshore) Low Pb (offshore)

Ave

rag

e la

rval

g

row

th ( m

d-1)

3.5

4.0

4.5

5.0

5.5BP = 0.23 P = 0.025

P = 0.76 P = 0.01

P = 0.01High Pb (nearshore) Low Pb (offshore)

Recruits from offshore had significantly wider metamorphic bands than did recruits from nearshore

Hindcast Age (days prior to settlement)

051015202530354045

Oto

lith

incr

emen

t wid

th ( m

d-1

)

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

High Pb (nearshore)Low Pb (offshore)

Recruits from offshore had grew significantly faster at the end of larval life than did recruits from nearshore

Condition (metamorphic band width, m)

10 15 20 25 30 35 40

Nu

mb

er o

f in

div

idu

als

0

5

10

15

20

25

10 15 20 25 30 35 400

5

10

15

20

25

High Pb (nearshore)Low Pb (offshore)

A) Recruits

B) Survivors

Recruits from nearshore (local retention) settled in high numbers (45% of total), but tended to be in lower condition. They subsequently suffered much higher selection intensity, and comprised only 23% of the recruit cohort after a month.

The intensity of selection was much greater for fish that The intensity of selection was much greater for fish that developed in nearshore than offshore waters, potentially developed in nearshore than offshore waters, potentially because only exceptionally strong performers from offshore because only exceptionally strong performers from offshore waters made it back to reefs (nearshore larvae had it easy). waters made it back to reefs (nearshore larvae had it easy).

Given this potential for asymmetrical post-settlement Given this potential for asymmetrical post-settlement source-based survival, we may need to pay attention to source-based survival, we may need to pay attention to ‘realized connectivity’ on ecological scales, that is, the ‘realized connectivity’ on ecological scales, that is, the proportion of individuals from different sources that survive proportion of individuals from different sources that survive to reproduce.to reproduce.

We may also be wrong about the importance of local We may also be wrong about the importance of local retention – it can lead to large numbers, but they may not retention – it can lead to large numbers, but they may not survive well after settlement.survive well after settlement.