Estuaries and Fish Ecology

Tim EssingtonSchool of Aquatic and Fishery

Sciences

Outline

• Fish diversity• What do fish use estuaries for, why?

– Dynamics of fish populations– Predation and Predation risk– Fish life histories and ontogenetic shifts

• Estuaries as nurseries

What do fish use estuaries for?

• Estuarine residents• Saltwater spawners

– Active and passive processes move larvae into estuaries

– Weakfish migration up the Hudson River• Estuarine spawners • Anadromous / Catadromous species

Boccacio Rockfish

Max. Size, 75-91 cm, Max weight = 6.8 kgLife Expectancy 50 – 100 yearsInternal live bearer (20,000 – 200,000 eggs)

Pacific HalibutReach sizes up to 2.5 m, > 300 kgLive approximately 30 yearsFecundity: 100,000–2,800,000 per year

Atlantic Silverside

Maximum size of 15 cm“Annual” species, mature at age 1, few survive to age 2Fecundity = 5,000 – 13,000 eggs

Spiny Dogfish

Maximum age ~ 75 yearsMature at age 30OvoviviparousPups are 18 – 30 cm at birthFemales produce fewer than 10 pups over a 2 year period

Why use estuaries?

• Productivity?• Refuge from predators?• Stability / predictability?

The m / g rule and ontogenetic habitat shifts

Werner and Gilliam, 1984. Ann. Rev. Ecol. Syst. 15: 393-425.

Predation Happens in Limited Time / Spatial “Arenas”

Implications: Habitat use of small fish is restricted

Promotes density dependent growth and survivorship

Decouples fish from their own food (local prey depletion)

Johan Hjort and “Year- Class Strength

Used scales to assess cohort-strength of norwegian herring

Realized that population dynamics were driven by variable year-class strength

Hypothesized that year-class strength was likely determined by survivorship through a critical period

This critical period is likely to occur very early in a cohort’s life (egg / larvae)

Simple model of bipartite life historyPrevailing Advection Field

Unsuitable habitat

Suitable Habitat

Value of Estuaries to US Fisheries

Nursery Role of Estuaries

Movement of larval stages into estuaries

Rearing in estuarine habitats

Movement to coastal ecosystem

Beck et al: What is a “Nursery Habitat”

• Context: long history of considering estuaries as critical / essential nursery habitats– Deegan 1993: Fish migration can be a significant

route of nutrient and energy flux• What makes a habitat a “nursery habitat”

– Importance judged by per-area production to adult stages

4 key processes:

Density (habitat selection)GrowthSurvivorshipMovement

Gillanders et al:

• What is evidence of movement between nursery-> non nursery habitats?

• What methods are used?• What are the scales of movement?

Web of Science Search “Otolith microchemistry”

Eelgrass (Zostera marina) in Puget Sound

Heck et al.: Density, survivorship and growth

• Meta-analysis of each demographic rate– Compares seagrass meadows to other habitats– Unstructured habitats routinely have lower:

• Density• Survivorship• Growth

– BUT: seagrass meadows were not especially “better” than other structured habitats

Questions:• Why are estuaries so commonly thought of as nursery

habitats? • Is the operational definition of ‘nursery habitats’ necessary,

and if so in what contexts?• How might these definitions account for the dynamic

contribution of estuaries to coastal populations?• Fisheries are one “ecosystem service” that estuaries provide.

How might this be quantified?• What anthropogenic changes are most likely to threaten fish

and invertebrate populations, and why? What might be important interactive effects?

Intensity of Seasonal Hypoxia and Springtime Conditions

Data from HCDOP Citizens Monitoring Program

Hypoxia Impacts in Hood Canal

Seasonal hypoxia

unimpacted

Hypothesized Effects

• Persistent effects: those present in Hoodsport even when DO is high– Demographic

• Immediate effects: those present in Hoodsport that are only manifest during hypoxia– Behavioral

General Results

Hoodsport Hazel Point Possession Sound Useless Bay

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Hoodsport Hazel Point Possession Sound Useless Bay

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June September

Hypoxia Impacts: Log response ratios to capture chronic vs. acute responses

Benthic Fish

Bentho-Pelagic Fish

Mobile Invertebrates