Marine Mammal Distribution

1. Importance of Understanding Distribution

  • Core of ecology & conservation:

    • Understanding where and when animals occur helps manage impacts from human activity.

    • Essential for:

      • Marine Protected Area (MPA) design.

      • Offshore wind farm & shipping management (reducing overlap with animals).

  • Challenges:

    • Marine mammals travel large distances.

    • Spend most of their time underwater → hard to detect.

    • Observation is limited and biased by method and conditions.

2. Ecological Drivers of Distribution

A. Reproduction

Reproductive behaviour has a profound effect on distribution because females need to protect calves and access suitable environments for birthing and rearing.

Case Study 1: Harbour Porpoise (Phocoena phocoena)

  • Usually solitary → pairs during mother–calf period.

  • Methods used: Strandings data (historically the main method before at-sea surveys).

  • Findings:

    • Strandings concentrated off west Denmark in the southern North Sea.

    • Most strandings occurred in summer (April–September), especially late summer, with many calves.

    • Suggests females move into calmer, warmer, sheltered areas for calving.

    • Area chosen likely due to low wind, low shipping, and protection from predators.

Case Study 2: Bottlenose Dolphin (Tursiops truncatus)

  • Populations:

    • NE Scotland (Moray Firth)

    • Cardigan Bay, Wales

  • Method: Photo-ID tracking of individuals (long-term social studies).

  • Findings:

    • Birthing peaks in summer months.

    • Mothers with calves avoid areas of high vessel traffic and offshore regions.

    • Preference for calm, sheltered coastal waters for protection.

Case Study 3: Baleen Whales (e.g., Blue Whales)

  • Long-distance migrants:

    • Feed in polar regions in summer, breed in tropical/subtropical waters in winter.

  • Drivers:

    • Warmer waters for birthing (easier calf survival).

    • Avoidance of predators (e.g., killer whales in higher latitudes).

  • Method: Satellite tagging.

  • Patterns: Seasonal north–south migrations with predictable calving grounds.

Case Study 4: Pinnipeds (Seals)

  • Behaviour: Haul out on land for birthing, nursing, and mating.

  • Males join haul-out sites after females give birth.

  • Central Place Foragers:

    • Must return repeatedly to feed pups (similar to seabirds feeding chicks).

  • Example: Tracked seals off SW Ireland show repeated foraging trips radiating from haul-out sites.

  • Seasonality varies by species (e.g., grey vs. common seals).

  • Key point: Reproduction drives aggregation and constrains movements to specific coastal zones.

Summary:
Reproduction → drives animals to safe, warm, sheltered, or predator-free habitats, often causing seasonal aggregations or migrations.

B. Foraging and Feeding

  • Essential for growth, reproduction, and survival.

  • Marine mammals have high energy demands → strong overlap with prey abundance.

  • Typical prey:

    • Crustaceans (krill).

    • Cephalopods (squid, cuttlefish).

    • Schooling fish (herring, sand eels, mackerel).

  • Prey availability and exploitability shape marine mammal distributions.

Seasonal Productivity: The Spring Bloom

  • Driven by sunlight and nutrient mixing.

  • Winter: High nutrients, low light.

  • Summer: High light, low nutrients.

  • Spring/autumn: Best balance → high productivity → food web surge → whales follow prey.

  • Lag effect: Plankton bloom → fish response → whales arrive shortly after.

  • Observed pattern: Peaks in whale sightings follow the spring bloom.

Persistent Productive Habitats

  • Some areas sustain productivity outside seasonal blooms due to oceanographic processes:

    • Shelf edges: Currents upwell nutrients.

    • Tidal fronts: Mixing of stratified and mixed waters.

    • Internal waves: Bring nutrients up from deep waters.

  • These sites (e.g., Celtic Shelf, NW Scotland) are long-term feeding hotspots.

Prey Spawning & Migration

  • Many fish species (e.g., herring, mackerel, capelin) have spawning migrations or habitat-specific egg-laying.

  • Example 1: Capelin (NW Atlantic):

    • Spawn in cool, coarse sediments, often nearshore.

    • Marine mammals target these spawning aggregations.

  • Example 2: Mackerel (NE Atlantic):

    • Seasonal migration: spawn in south, feed in north, overwinter south.

    • Mammals track these prey migrations along the shelf edge.

Summary:
Foraging patterns are linked to primary productivity, prey aggregation, and migrations → predictable seasonal distributions.

3. Methodological Factors (How We Observe Distribution)

A. At-Sea Surveys

Most common approach to document marine mammal distributions.
Two main types:

  1. Vessel-based surveys

  2. Aerial surveys

Methods

  • Strip Transects:

    • Fixed-width area.

    • Assume all animals within strip are detected.

    • Useful for small areas, straightforward density calculations.

  • Line Transects:

    • Observers record distance from transect line.

    • Allows variable detection range.

    • More flexible, accounts for declining detectability with distance.

Metrics:

  • Encounter rate: animals per km travelled.

  • Density: animals per km² surveyed.

Examples

  • SCANS Surveys (European gold standard):

    • Pan-European aerial and vessel surveys.

    • Conducted ~every 10 years (1994, 2005, 2016, 2022).

    • Provide large-scale snapshots of distribution and abundance.

    • Limitations: infrequent (temporal gaps), expensive (~£30k/day for large vessels).

  • Ferry/Volunteer Surveys (e.g., ORCA):

    • Continuous data from ferries (“vessels of opportunity”).

    • Advantages: Excellent temporal coverage.

    • Limitations: Poor spatial coverage (fixed routes).

    • Best used to complement large-scale surveys.

B. Detection Biases
1. Availability Bias

  • Animal is underwater → not available for detection.

  • Varies by species (e.g., dolphins visible; deep-divers like fin whales rarely at surface).

2. Perception Bias

  • Animal is available but not detected due to observer limitations or conditions.

  • Influenced by:

    • Sea state and visibility (easier in calm seas).

    • Behaviour (splashing dolphins vs. shy porpoises).

3. Attraction Bias

  • Animals may change behaviour due to survey platform:

    • Positive bias: Dolphins attracted to boats (bow-riding).

    • Negative bias: Porpoises/whales avoid vessels.

  • Depends on platform type and speed (planes unaffected).

C. Survey Conditions

  • Surveys biased toward summer months → calmer, safer conditions.

  • Winter surveys rare → seasonal bias in data.

  • Example: Welsh dataset shows peak survey effort in summer.

  • Implication: Maps often reflect summer distributions, not year-round presence.

4. Observed Distributions of Key European Marine Mammals

Dataset:

Combined vessel, aerial, and digital surveys across UK & Ireland (Moore & Page, ~10-year project).

Common Dolphin (Delphinus delphis)

  • Large, sociable pods; lively surface behaviour.

  • Found mainly west of UK, offshore, year-round.

  • Moves closer inshore during summer.

Striped Dolphin (Stenella coeruleoalba)

  • Similar ecology, but restricted to southern waters (Bay of Biscay, Iberia).

  • Offshore species, little northward movement.

  • Distinct north–south gradient vs. common dolphin.

Fin Whale (Balaenoptera physalus) & Minke Whale (B. acutorostrata)

  • Both move northward in summer (feeding season).

  • Fin whale → mostly offshore, southern (Bay of Biscay).

  • Minke whale → more inshore, around UK shelf waters.

  • Seasonal increase in summer sightings, drop in winter.

Pilot Whale (Globicephala melas) & Risso’s Dolphin (Grampus griseus)

  • Both feed mainly on cephalopods.

  • Pilot whales: Offshore, concentrated along shelf edge.

  • Risso’s dolphins: More coastal, common around Anglesey/NW Wales.

White-Beaked & White-Sided Dolphins

  • Both northern, fish-eating species.

  • White-beaked dolphin: Inshore/shelf species.

  • White-sided dolphin: Offshore, north of Scotland.

Bottlenose Dolphin (T. truncatus)

  • Two ecotypes:

    • Coastal (e.g., Cardigan Bay, Moray Firth).

    • Offshore (majority of population, at shelf edge W of Ireland).

  • Offshore ecotype resident year-round.

Killer Whale (Orcinus orca)

  • Small, well-studied coastal pods (e.g., Shetland, Hebrides).

  • Majority of population offshore, feeding on pelagic fish (herring, mackerel).

  • Different diet/behaviour from coastal seal-feeding pods.

Sperm Whale (Physeter macrocephalus)

  • Only males in northern hemisphere waters.

  • Found along deep shelf edge, feeding on squid.

  • No clear seasonality detected.

Harbour Porpoise

  • Inshore species (<400 m depth).

  • Strong presence in southern North Sea, esp. Denmark–UK corridor.

  • Seasonal northward shift:

    • Winter: southern North Sea.

    • Summer: northern North Sea (following sand eel emergence).

  • Distribution has changed in last 30 years → major movement southward since 1990s.

  • Critical for MPA planning and conservation focus.

5. Summary & Takeaways

  • Distribution = Ecology + Observation Method.

  • Ecological drivers:

    1. Reproduction: Need for protection → seasonal aggregation or migration.

    2. Foraging: Driven by productivity, prey aggregation, and spawning cycles.

  • Methodological drivers:

    • Survey type, detection bias, and seasonal timing shape the “observed” distribution.

  • Patterns among species:

    • Clear inshore vs. offshore, north vs. south, and summer vs. winter differences.

  • Conservation relevance:

    • Mapping seasonal and spatial use helps identify Important Marine Mammal Areas (IMMAs).

    • Understanding distribution shifts is crucial for adaptive management as climate and human activity change.