Oscar Belloso, Alexandra Urdaneta, Ana Elena Sanchez and Claudia Martino

CORAL REEF: 2

Variation in fish density, assemblage composition and relative rates of predation among mangrove, seagrass and coral reef habitats.

INTRODUCTION

Introduction to the investigation: Key vocabulary, the purpose and some background information.

KEY WORDS• Assemblage: A collection of gathering things.

• Nursery: A place or habitat that breeds or supports animals.

• Mangrove/Seagrass Beds: Mangroves and seagrass beds are shallow-water habitats that often have greater abundance of juvenile fishes to coral reefs. They have been referred to as nursery habitats because of the apparent increase in the survival of young individuals that these abundance patterns suggest.

BACKGROUND INFORMATION• Null Hypothesis: There is no significant difference in nursery function among mangrove, seagrass

and shallow reef habitat as measured by: (a) patterns of juvenile and adult density, (b) assemblage composition, and (c) relative predation rates.

• The results in this research suggested, that although both mangrove and seagrass showed characteristics of nursery habitats, this pattern was weak.

• Very few species showed patterns that were significant. They were half of the mangrove and seagrass sites which appeared to hold higher numbers of juvenile fish.

• Only four of the six most abundant and commercially important species (on research paper) showed higher numbers of juvenile fish in mangrove and/or seagrass habitats relative to coral reefs.

• Although mangrove and seagrass had very low predation risk, this was not the case for all of them.

• The results caution against generalization that all mangrove and seagrass have nursery function.

PURPOSE OF INVESTIGATION• Coastal habitats such as coral reefs, mangroves and seagrasses beds are amongst the most diverse.

• Coastal habitats have very important scientific and economic reasons in vital ecosystem services:

✓ Nutrient cycling

✓ Waste treatment

✓ Disturbance regulation

• This study was to decide whether mangrove and seagrass beds of the Caribbean are more important for reef fish species by testing the null hypothesis that there was no difference in nursery value among mangrove, seagrasses and coral reef habitats.

• It is important to understand that there are many factors that affect the distribution and abundance of organisms among coastal habitats.

• This investigation informs how these factors affect the distribution of species.

• Mangrove and seagrass are characterized in having shallow habitats that are often to observe to have greater abundance of juvenile fishes compared to coral reefs.

• These habitats that are referred as nurseries have the following characteristics:

• They have an increase in juvenile survival.

• There are several factors that need to be considered in evaluating the value of a nursery:

✓ Growth

✓ Survival

✓ Density

✓ Movement

INTRODUCTION OF INVESTIGATION

• There are factors that affect organism survival that is mention numerously in this investigation:

• Competition

• Predation

• This factors will vary to which the organism settles.

• These factors increases the probability of an organism’s survival due to the supply of food and shelter as well as the supplement of nearby populations.

• They conducted this study in three locations:

• Lee Stocking Island, Great Exuma

• Bahamas

• Mahahual, Mexico

• Turneffe Atoll, Belize

SITES OF EXPERIMENTATION

EXPERIMENTAL DESIGN

Experimental design, materials, procedure and variables of the experiment.

PROCEDURE1. One site from each habitat type was visually censused to find the abundances of juvenile and adult fishes.

✓ The juvenile and adult status was determined from a combination of size range and color patterns based on published life history data.

✓ Fishes that were within 30 m2 of the transects were recorded.

1. The proportional abundance of juveniles among habitats was compared to test the null hypothesis of no differences among habitats in terms of nursery potential.

✓ The proportional abundance of juveniles among habitats at the assemblage level was compared and then, the proportional abundance of juveniles among habitats for each of the most abundant and commercially important species were compared.

1. The null hypothesis of no difference in relative predation rates between habitats was tested by examining the removal times of tethered fish.

✓ A live , juvenile smallmouth grunt (ranging in sizes from 3 to 6 cm total length) was used as baitfish in timed experiments within mangrove and seagrass beds.

✓ A fish was tethered to active the stopwatch tethered timers to assess the relative rates of predation: a) between habitats b) between morning and afternoon c) between sites

✓ 8 tethered baitfish were placed in an area of devoid predators

PROCEDURE1. DENSITY

• The proportional abundance of juveniles among habitats was compared using:

✓ Univariate ANOVA was used to compare proportional abundance of juveniles among all three habitats at each site.

✓ If necessary, this was followed up by Bonferroni post-hoct tests.

• Two levels of biological resolution were investigated:

1. Proportional abundance of juveniles among habitats at the assemblage level

2. Comparisons of the proportional abundance of juveniles among habitats for each of the most abundant and commercially important species

2. ASSEMBLAGE COMPOSITION

• Using Jaccard’s coefficient of similarity, the null hypothesis that species are not shared among habitats was tested.

3. RELATIVE PREDATION RATES

• The null hypothesis of no difference in relative predation rates between habitats was examined by analyzing the removal times of tethered fish:

✓ A smallmouth grunt: used as baitfish in timed experiments within mangrove/seagrass beds and coral reefs at each of two sites.

➡ Several smallmouth grunt were collected using a gill net, monitored for stress for 20 minutes and then attached to a tether timer by placing a hook through the dorsal musculature.

➡ The length for each fish was recorded.

3. The tether timer was started and the timer with the batifish was placed in a given habitat.

➡ A stopwatch with a chronometer was fixed inside a 30 ml nalgene container and a magnetic switch was attached to the start/stop function of the watch.

➡ Fish were tethered with a monofilament line, which in turn was connected to the start/stop switch.

➡ If a fish was taken the switch would open, causing the watch to stop.

➡ At the end of 90 minutes, experiments were concluded and the tether timers were collected.

Control - Relative Predation Rates Preliminary Exam

Conducted to ensure that the movement of the baitfish would not accidentally stop

the clock.

✓ Eight tethers, each with baitfish that were 6 cm in total length, were placed in an area devoid of predators.

✓ The baitfish were visually monitored for 10 minutes with an observer 3 m away.

✓ Observations confirmed that batfish removals or clock stoppages were not the result of fish behavior.

Independant Variable - Amount of time (90 Minutes)

Dependant Variable - The survival of the fish

MATERIALS• Univariate ANOVA (computer program)

• Tethered fish (Haemulon chrysargyreum or smallmouth grunt)

• Gill net

• Hook

• Stopwatches tethered timers

• Container

• Statistical software package

DATA PROCESSING & COLLECTION

The collection of data, how to process the data and the analysis.

ASSEMBLAGE COMPOSITION

• If mangrove and seagrass habitats are nurseries, then they are expected to share species

• Tested null hypothesis:

‣ species not shared between habitats ≈ lack of nursery potential

HOW THEY DID IT:

1. Used transect sampling to record all the fishes if observed within 30 cm2

2. Tested the null hypothesis by examining similarity in assemblage structure among habitats (censuses)

JACCARD’S COEFFICIENT OF SIMILARITY

Sj =c

(a+b+c)

Cnumber of species that are in both

habitats

Anumber of species present in habitat

A

Bnumber of species present in habitat

B

• 7,200 fishes - 82 species

• Few species in common

• Between reef and mangrove:

• Sj = 0.22 +/- 0.18

• Between reef and seagrass:

• Sj = 0.23 +/- 0.20

RELATIVE PREDATION RATES

• If tethered fish was taken = switch would open, watch would stop

• Timers were started and placed in a given habitat

• Placed 3-8 replicate tether timers

• Experiments concluded at end of 90 min.

• Tethering experiments constrain behavior of the baitfish = can only assess relative risk of predation

RELATIVE PREDATION RATES

CONCLUSION AND EVALUATION

Population density, discussion and final comments.

DENSITY

Null hypothesis was rejected:There is a difference in nursery function among mangrove, seagrass

and coral reef habitat

BUT it doesn’t mean that mangroves & seagrass provide nursery function.

Turneffe Atoll Site #6

Turneffe Atoll Site #3

higher abundance of juveniles in mangroves

higher abundance of juveniles in reef and seagrass

Analysis says: only 7/54 occasions demonstrated that there was a higher abundance of juveniles in mangroves and/or

seagrass than coral reefs

ASSEMBLAGE COMPOSITION

Null hypothesis was rejected:

Only 11 of the most abundant species were considered

Jaccard’s Coefficient of Similarity → few species were in common between:

Reef and Mangrove (0.22) Reef and seagrass (0.23)

Then, only the most abundant AND commercially important species were considered

Jaccard’s Coefficient of Similarity → few species were in common between:

Reef and Mangrove (0.25) Reef and seagrass (0.20)

RELATIVE PREDATION RATES

Null hypothesis was rejected:

Average survival rate of baitfish in mangrove/seagrass beds: 42%Average survival rate of baitfish in coral reefs: 15%(after 90 minutes of being tethered)

SITE 3

Morning AfternoonMangrove/Seagrass - 33% morality within 90 minutesCoral reefs - 100% mortality within 80 minutes

Mangrove/Seagrass - 100% within 78 minutesCoral reefs - no significant change between morning and afternoon results

Evaluation: Tethering experiments contain behavior of baitfish

DISCUSSION

Juvenile survivorship = 2.7 times greater in mangrove/seagrass habitats than coral reefs

Evaluation: Conclusion identifies the need of similar studies to be conducted that replicate habitats and thus better assesses nursery function

Mortality of individuals varied between sites and time of the day.

Results:Population density, assemblage composition and relative rates of predation show that not

all mangrove and seagrass offer nursery function (generalization must be avoided)

Evaluation: A more cautious and explicit analysis of ecological roles of such

habitats is required

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