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Patterns of habitat utilization by deep-sea fish in the Bay of Biscay, NE Atlantic

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2. Material and Methods

2.1. Field and image-based observations

The four dives analysed in this study took place in 1998 during a cruise in the Bay of Biscay performed by the submersible “Nautile” of IFREMER, where the main objective was to make direct underwater observations on fishes and invertebrates of commercial interest and the impact of fishing gears. The details of the sampling were given by Latrouite et al. (1999) (Figure 1, Table 1). The submersible cruised 1.5 m above the sea bottom at variable speed, normally within the range of 0.5-0.7 knots. The estimated visual field was about 10 meters wide (5 m on each side) and 10-15 m long (ahead), depending on the turbidity of the water, plankton density, and seabed topography. Both a fixed camera and a mobile camera were operated to record video sequences of the area covered, and navigation and physical records were kept automatically. The methodology differed from normal transect sampling because the submersible made frequent stops to study in detail the habitat surrounding individual fish.

 

Table 1. Dive transects main characteristics.

Dive 22

Dive 34

Dive 35

Dive 37

Date

17-05-1998

29-05-1998

30-05-1998

01-06-1998

GPS Position

47°54'N 08°11W

44°43'N 02°09W

46°15'N 04°34'W

47°28'N 06°41'W

Distance crossed (meters)

3600

3800

5100

5270

Time hours

10:57-15:41

09:56-14:32

09:44-14:20

10:14-14:20

Duration (minutes)

219

275

274

246

Depth (meters)

931-1301

710-1561

1153-1561

422-538

Dive path inclination

Variable

High

Low

Low/Medium

Temperature

7.2-9.6

6.2-10.3

4.6-9.1

10.9-11.6

Bioturbation

Bottom type

Current

High

Soft sediment

Variable

Medium

Fine sediment

Constant

None

Hard/Structured

Variable

None

Soft/Mixed

Constant

 

Sampling units in the present study were individual fish. Other macro-nekton (e.g. cephalopods and crustaceans) were only sporadically observed and were not considered in the analysis.

In the laboratory, videotapes were analysed using a Super VHS video player and a large screen, or processed by means of computer and video with software Microsoft Media Player©®. Recording of habitat factors started immediately after an individual fish appeared in a video sequence. Traits such as size, shape and colour were used for identification of the fish and bottom epifauna. Even under good conditions of light and turbidity only individuals or colonies larger than about 5 cm could be suitably recognized. These were identified to the lowest possible taxonomic level. Microhabitat of each individual was characterized according to depth, temperature, slope inclination, bottom type and complexity, current velocity, benthos type and cover (Tables 1, 2, 3). In this study, microhabitat refers to the ensemble of biotic and abiotic conditions of the fish surroundings.

 

Table 2. Number of individuals per species per dive transects and codes in brackets.

Fishes Dive 22 Dive 34 Dive 35 Dive 37
Alephocephalidae (Ale) 0 2 5 0
Anguiliformes (Ang) 12 5 18 1
Beryx decadactylus (Ber) 0 0 0 10
Chimaerids (Chi) 7 4 4 36
Coryphaenoides rupestris (Cor) 61 7 103 0
Galeus melastomus (Gal) 0 0 0 0
Helicolenus sp. (Hel) 0 0 0 41
Hoplostethus atlanticus (Hoa) 4 1 334 0
Lepidion eques (Lep) 52 16 12 0
Mesopelagic fish (M) 7 3 0 0
Macrouridae - others (Mac) 15 30 10 5
Mora moro (Mm) 18 4 0 0
Molva molva (Mol) 0 0 0 40
Moridae - others (Mor) 6 2 10 0
Neocyttus helgae (Neo) 0 3 14 0
Notacanthus sp. (Not) 1 3 10 23
Sharks (others) (Sha) 3 3 29 1
Synaphobranchus kaupii (Syn) 57 88 26 0
Trachyscorpia sp. (Tra) 8 0 0 0
Total 251 171 575 157
 

Table 3. Categories used to define the fish habitats.

Variable

Type (unit) or categories

Depth

Continuous (m)

Temperature

Continuous (°C)

Current (Cur)

1. Absent/very low; 2. Low/moderate; 3. High/very high

Slope inclination (Slo)

1. Flat (0-5°); 2. Sloping (5-30°); 3. Steeply sloping (30-45°); 4. Steep (>45°)

Substrate Complexity (Sub)

1. Sedimentary; 2. Structured; 3. Complex

Ripple marks (Rip)

0. None; 1. Weak; 2. Strong

Bottom type

1. Coarse sediment; 2. Fine sediment

Bottom Structure (Bot)

Coverage by clast, rock or hard bottom: 1. (0-5%); 2. (5-25%); 3. (25-50%); 4. >50%

Benthos cover

Actinians (Act), Asteroidea (Ast), Pennatularians (Pen), Crinoids (Cri), Echinoids (Ech), Bryozoans (Bry), Hydroids (Hyd), Sponges (Spo), Scleractinians (Scl), Gorgonians (Gor), Antipatharians (Ant), Sea Cucumbers (Sea), Without benthos (Des)

 

Following a description of microhabitat use in single dives, a global dive analysis was attempted using multivariate techniques. Owing to the mixed nature of the variables (discrete and continuous), Canonical Correspondence Analysis (CCA) as implemented in the CANOCO 4© software (Leps & Smilauer 2003; Ter Braak & Smilauer 1998) was considerate appropriate for the exploratory analyses. The environmental variables of greater influence in the model fit were chosen by means of automatic forward selection using Monte-Carlo permutation tests (F-test: significance level set at alpha=0.05). A Monte-Carlo permutation test is a test of statistical significance obtained by repeatedly shuffling (permuting) the samples (Leps & Smilauer 2003). 

 
2.2. Statistical analyses

Prior to the analyses, the species, physical and environmental variables were coded according to the nomenclature given in Tables 1, 2 and 3. Stratified re-sampling of the original observation matrix was required to balance the number of observations between dives and fish species.120 observations were then randomly selected for each transect, and a matrix containing 4 x 120 individuals used for the global analysis.

Similarly, only the 11 most common types of epifauna, as well as one variable coding for absence of epifauna, were used as environmental variables, in order to down weight the occurrence of rare associations. A more detailed description of the numerical analyses is available in Mendes Alves (2003).

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