Sharks in the SoundHow An Apex Predator Chooses Habitat

Prepared by Charles Bangley, Coastal Resources Management Program, East Carolina University

Scott Taylor

Habitat Selection

Organisms choose habitat based on abiotic and biotic factors

Abiotic Factors – local environmental conditions affecting habitat choice

Depth, salinity, temperature, dissolved oxygen

Biotic Factors – biological processes that influence habitat choice

Prey availability, predator presence, reef-building organisms

Abiotic and biotic factors can influence each other

Examples – prey migrations triggered by temperature, low oxygen from eutrophication

NC Coastal Reserve Emily Woodward

What are the Abiotic and Biotic Factors in this Image?

Resource Partitioning

What happens if two species have the same habitat preferences?

Competition is strongest between species with similar habitat use behavior

Resource Partitioning – species use the same habitat differently to avoid competition

Two types:Temporal – species use the habitat area at different timesSpatial – species use different areas of the habitat

Alexander Safonov

ECU Inshore Shark Survey

Research Goals – Identify habitat preferences of coastal sharks in NC estuaries, determine if resource partitioning is occurring

Methods – Capture, identify, measure sharks using gillnet, longline, and drumline gear, take environmental measurements

Data – Shark catch and environmental data from Back Sound, North Carolina

Maria de Oca Cecilia Krahforst

The Data

Species – species of shark caught at the environmental values in that row

Abiotic factors – depth (m), temperature (°C), salinity (ppt), dissolved oxygen (mg/L), distance from the nearest inlet (km)

Biotic factors – distance from the nearest seagrass habitat (m)

Charles Bangley

The Sharks

Cecilia Krahforst

Blacknose shark (Carcharhinus acronotus)

Cecilia Krahforst

Blacktip shark (Carcharhinus limbatus)

Steven Licthi

Atlantic sharpnose shark (Rhizoprionodon terraenovae)

Meganne Rose

Bonnethead shark (Sphyrna tiburo)

Identifying Habitat Preferences

Habitat preferences identified by finding mean environmental values for each species

Mean – the average environmental value for that species

Many samples won’t fall exactly on the mean, we need to account for that

Standard Deviation – usual range of difference from the mean within your samples

Usually reported as Mean ± Standard Deviation

Mean of 5 ± 2 = average value is 5, but values from 3 to 7 are within the usual amount of variation in your samples

Also useful to know the minimum (lowest) and maximum (highest) values for each species

Charles Bangley

Basic Stats in Excel

Microsoft Excel will calculate some statistics using formulas

Step 1 – Sort the data by Species.

Step 2 – Type the formula into a blank cell.

Step 3 – highlight cells in the column containing the factor you’re analyzing. Make sure to only highlight those next to the species name you’re working on.

Step 4 – Press Enter or Return to run the formula.

Mean: =AVERAGE(Top cell:Bottom cell)

Formulas

Standard Deviation: =STDEV(Top cell:Bottom cell)

Minimum: =MIN(Top cell:Bottom cell)

Maximum: =MAX(Top cell:Bottom cell)

Analysis Questions

1. What do the statistical values tell you about the habitat preferences and behavior of each species?

2. What environmental variables differ between species?3. How might habitat use differences allow these sharks to coexist in the same

estuary? 4. Based on habitat preferences, which of these sharks might be most affected by

habitat loss?

Steven Licthi