Marine Resources Assessment for the Marianas Operating ... - SPREP

AUGUST 2005 FINAL REPORT
4.0 FISH AND FISHERIES
4.1 FISH/INVERTEBRATES
Distribution and abundance of fishery species depends greatly on the physical and biological factors
associated with the ecosystem, as well as the individual species. Physical parameters include habitat
quality variables such as salinity, temperature, dissolved oxygen, and large-scale environmental
perturbations (e.g., ENSO). Biological factors affecting distribution are complex and include variables
such as population dynamics, predator/prey oscillations, seasonal movements, reproductive/life cycles,
and recruitment success (Helfman et al. 1999). Rarely is one factor responsible for the distribution of
fishery species, but is a combination of factors. For example, pelagic species optimize their growth,
reproduction and survival by tracking gradients of temperature, oxygen, or salinity (Helfman et al. 1999).
Additionally, the spatial distribution of food resources is variable and changes with prevailing physical
habitat parameters. Another major component in understanding species distribution is the location of
highly productive regions such as frontal zones. These areas concentrate higher trophic-level predators
such as tuna and provide visual clues for the location of target species for commercial fisheries (NMFS-
PIR 2001).
Environmental variations, such as ENSO events, change the normal characteristics of water temperature,
thereby changing the patterns of water flow. The NEC (westward) and the Subtropical Countercurrent
(eastward) are major influences on distribution of fishes and invertebrates in the Marianas MRA study
area and vicinity (study area; Eldredge 1983). ENSO events alter normal current patterns, alter
productivity, and have dramatic effects on distribution, habitat range and movement of pelagic species
(NMFS 2003a).
In the northern hemisphere, El Niño events typically result in tropical, warm-water species moving north
(extending species range), and cold-water species moving north or into deeper water (restricting their
range). Surface-oriented, schooling fish often disperse and move into deeper waters. Fishes that remain
in an affected region experience reduced growth, reproduction, and survival (NOAA 2002). El Niño events
have caused fisheries such as the skipjack tuna fishery to shift over 1,000 km (NMFS-PIR 2001).
Coral reef communities surrounding the study area have a reputation for year-round uniformity and
stability (Amesbury et al. 1986). While this is true for most species in the area, there are exceptions.
Seasonal variations in pelagic species distributions in the area are understood. Several of the reef fish
species (juvenile rabbitfish, juvenile jacks, juvenile goatfish, and bigeye scad, Selar crumenophthalmus)
targeted in the study area show strong seasonal fluctuation, usually related to juvenile recruitment
(Amesbury et al. 1986).
Fish species composition within the study area is typical of what you find in most Indo-Pacific insular,
coral reef-bordered coastal areas. Seventy-three percent of the total number of species found belongs to
20 families (Myers and Donaldson 2003). The geographic location of the study area suggests a more
diverse ichthyofauna than areas such as the Hawaiian Islands. Recorded species diversity in the
Guam/Marianas island chain is lower than that of the Hawaiian archipelago. Actual diversity may be
higher and the recorded diversity may be an artifact of insufficient sampling (Paulay 2003a). However,
many other factors, such as larval recruitment and frequent natural disturbances, have dramatic impacts
on species diversity (Randall 1995). Myers and Donaldson (2003) noted the occurrence of 1,019 fish
species (epipelagic and demersal species found to 200 m) within the study area. Inshore species are
composed primarily of widespread Indo-Pacific species (58%) with the remainder consisting of
circumtropical species (3.6%) and nearly equal numbers of species with widespread distributions primarily
to the west, south, and east of the islands (Myers and Donaldson 2003). Ten species of inshore and
epipelagic fishes are currently considered endemic to the Marianas. However, this number is probably too
high due to the observations of transient species in the area (Myers and Donaldson 2003). Additionally,
Myers and Donaldson (2003) identified 1,106 species of fish known from the Mariana Islands and
adjacent territorial waters. Extensive studies have been done on the biogeography of inshore and
epipelagic fauna found in the Marianas from 0 to 100 m. Currently, occurrence and distribution of benthic
and mesopelagic species from 100 m to greater than 200 m are incomplete and poorly understood
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