Larval Fish In Estuarine And Coastal Waters

Nearly all fish have an early pelagic larval stage and thus comprise an interesting component of zooplankton samples. Most fish larvae hatch from pelagic or demersal eggs (that is, attached to sand, rocks or seaweed), but there are a few live-bearing species. The larval stage usually lasts 3 to 4 weeks. The presence of fish larvae can indicate important spawning areas, or fish biodiversity, and therefore larval diversity may have greater relevance than mere presence of a transient adult. For example, south-eastern Australia has a large diversity of fish in late summer, because of larval transport from the Great Barrier Reef, yet it is its role as a spawning location that is important for conservation efforts. The source, supply and sinks of larvae are vital components for managing fisheries and the establishment of marine parks (Box 8.10).

Based on their life history, the majority of fish larvae caught in estuaries can be categorised as estuarine or marine opportunists, with low numbers of freshwater or marine straggler species (Potter et al. 1990). Estuarine species, which are usually small as adults, spawn, and spend their whole life cycle, in the estuary. In contrast, estuarine opportunist species spawn at sea, usually in coastal waters, with the larvae entering estuaries where the juveniles settle into nursery habitats such as seagrass beds and mangroves. Adults of these

BOX 8.10 LARVAL FISH CONDITION AND DEFORMITIES

The larval stage of fish is considered a bottleneck for fisheries, through starvation of the larvae (insufficient nauplii as food), predation (from jellyfish, ctenophores, krill or fish) and unfavourable currents. Over 99% of the eggs and larvae do not survive, and therefore rapid growth may enhance survival by reducing the duration of the vulnerable larval stages. Consequently fisheries biologists estimate age and larval growth from the width of the daily growth increments of the earstone or otolith (which are analogous to tree rings). Even body width or weight are useful indicators of larval condition, in response to environmental conditions (water temperature or pollution). Fish larvae are very delicate -without scales - so they are susceptible to poor water quality from urban run-off, acidic water or sewage effluent. The incidence of deformities in newly hatched larvae, relative to a control group, is a useful measure of water quality.

species remain in estuaries, only leaving the estuary to spawn or permanently migrate out of the estuary to coastal reefs. Small numbers of larvae of freshwater and marine straggler species can also occur in estuaries depending on the degree of input of marine or fresh water into the system.

Estuarine-spawning species only a short life cycle of 1-2 years and have a number of reproductive strategies to reduce the mortality of eggs and larvae. These strategies include being live bearers, such as the pipefish and seahorses (sygnathids, where larvae develop in a pouch on the males), and the apogonids (mouth brooders) with juveniles hatching at an advanced stage of development. Gobies, blennies, hemiramphids and atherinids have benthic eggs that are attached to seagrasses or other hard substrates such as mollusc shells. Some herring, and other fish with pelagic eggs, spawn in the upper reaches of the estuary to minimise the chance of the eggs being washed out of the estuary. Such strategies mean that larvae hatch at an advance stage of development, which allows them to be retained within estuaries and not be carried out by ebb tides.

The abundance of larvae of estuarine species usually shows a seasonal pattern, with highest abundances in summer and the lowest in winter. The seasonal variation in abundance closely follows the cycle of water temperatures. The abundance of larvae of marine opportunist species entering estuaries also shows a similar, but less-marked, seasonal variation. This is due to the larvae of taxa such as sparids, girellids and scorpaenids entering estuaries during winter.

Species diversity of fish larvae generally decreases with increasing distance upstream, away from the mouth of the estuary. Although samples from estuarine stations usually have a much lower diversity compared with marine stations, abundances of larvae of estuarine species are usually much higher than for larvae of marine opportunist species entering the estuary on the flood tide (Neira and Potter 1994).

Most estuaries in southern Australia and southern Africa are microtidal, with a narrow entrance channel opening into a large basin or basins. Tidal movements can carry larvae into and out of the estuary. Surveys of larvae in these estuaries report higher abundances of larvae from marine-spawned eggs on flood tides and higher abundances of larvae from estuary-spawned eggs on ebb tide, with higher abundances of larvae at night irrespective of flow direction (for example, Whitfield 1989; Neira and Potter 1992; Trnski 2001). Higher abundances of larvae in surface waters of estuaries at night are due to diel vertical migration of larvae through the water column. Possible reasons for diel vertical migration may be that it reduces predation risk or increases prey densities if larvae occur deeper in the water column during the day and near the surface at night.

Larval fish usually have a very different morphology compared with the pelagic or demersal adults, making them very difficult to identify. Recently, a number of larval fish identification guides have been produced that illustrate the development of larvae from different geographical regions. These identification guides have described larvae, to at least family level, of the majority of species that occur in estuarine and coastal marine waters (for example, Fahay 1983; Moser et al. 1984; Ozawa 1986; Okiyama 1988; Olivar and Fortuno 1991; Moser 1996; Neira et al. 1998; Leis and Carson Ewart 2000).

The most common method of identification of unknown fish larvae is the series method. This involves identifying the largest available larval or juvenile specimen in the samples, based or adult characteristics such as fin meristics and vertebral number (equivalent to the number of myomeres or muscle blocks - in larvae). The largest specimen is then linked to smaller specimens in the series by using morphological and pigment characteristics. A variety of characters can be used to identify fish larvae including general morphology, such as body shape, gut length and degree of coiling, number of myomeres, pigmentation patterns (melanaophores), the sequence of development of fins and the pattern of head spination (Table 8.2).

The length and stage of development (Box 8.11) are important features in identification. The gas bladder, which is present in many larvae is absent in adults (such as gobies). During the day, the gas bladder may be small in larvae, but strongly inflated and conspicuous in larvae caught at night, which can result in larvae of the same species appearing

Table 8.2. Key identification of features of fish larvae occurring in estuaries (based on information from Leis and Carson Ewart 2000 and Neira et al. 1998)

Family

Estuary (E) or Marine (M)

Features

Gobiidae (goby)

E/M

24-34 myomeres; body elongate to moderate; lightly to heavily pigmented; gut moderate and slightly coiled; conspicuous gas bladder. (Figure 8.12K)

Atherinidae (hardyhead)

E

35-47 myomeres; body very elongate; moderately pigmented; gut coiled and compact. (Figure 8.12D)

Hemiramphidae (garfish)

E

51-57 myomeres; body very elongate; moderately to heavily pigmented; gut very long. (Figure 8.12C)

Clupeidae (herring, sprat)

E/M

41-55 myomeres; body very elongate; lightly pigmented; gut very long. (Figure 8.12B)

Engraulidae (anchovy)

E/M

38-47 myomeres; body very elongate; gut very long; lightly pigmented. (Figure 8.12A)

Ambassidae (glass perchlet)

E/M

24-25 myomeres; body depth moderate; lightly pigmented; gut coiled and compact; conspicuous gas bladder; small preopercular spines. (Figure 8.12P)

Sygnathidae (pipefish, seahorse)

E

Elongate body; prominent dermal plates; moderately to heavily pigmented. (Figure 8.12J)

Blenniidae (blenny)

E

Typically 30-40 myomeres; body elongate; lightly to moderately pigmented; gut short and coiled; moderate to large teeth; none to large preopercular spines. (Figure 8.12I)

Gerreidae (silverbiddies)

M

24-25 myomeres; body depth moderate; lightly pigmented; gut moderate coiled and compact; prominent ascending premaxillary process; small preopercular spines. (Figure 8.12O)

Sparidae (bream, porgy, tarwhine)

M

24-25 myomeres; body depth moderate; lightly pigmented; gut moderate, coiled and compact; small to large preopercular spines. (Figure 8.12L)

Girellidae (blackfish)

M

26-27 myomeres; body depth moderate; lightly to moderately pigmented; gut moderate, coiled and compact; small preopercular spines. (Figure 8.12S)

Monacanthidae (leatherjacket)

M

19-20 myomeres; body deep and laterally compressed; moderately to heavily pigmented; gut moderate, coiled and compact; prominent dorsal and pelvic spine with barbs. (Figure 8.12E)

Monodactylidae (moonfish)

M

24 myomeres; body deep and laterally compressed; moderately to heavily pigmented; gut moderate, coiled and compact; large early forming pelvic fins; large preopercular spines. (Figure 8.12R)

Mugilidae (mullet)

M

24-25 myomeres; body depth moderate; heavily pigmented; gut long and coiled; small preopercular spines. (Figure 8.12U)

Platycephalidae (flathead)

M

27 myomeres; body depth moderate; moderately pigmented; gut moderate to long and coiled; large and early forming pectoral fins; extensive head spination. (Figure 8.12H)

Scorpaenidae (scorpionfish)

M

24-28 myomeres; body depth moderate; moderately pigmented; gut moderate to long and coiled; large early forming pectoral fins; extensive head spination. (Figure 8.12G)

Silliganidae (whiting)

M

32-45 myomeres; body elongate; lightly pigmented; gut moderate to long and coiled; very small preopercular spines. (Figure 8.12N)

Terapontidae (trumpeter)

M

25 myomeres; body elongate; lightly pigmented; gut coiled and moderate; small preopercular spines. (Figure 8.12M)

Callionymidae (dragonet)

M

20-22 myomeres; body robust and moderately deep; heavily pigmented; gut coiled and moderate to long; one large preopercular spine. (Figure 8.12T)

Paralichthidae (flounder)

M

33-39 myomeres; body moderately deep and laterally compressed; moderately pigmented; gut coiled and moderate to long; small preopercular spines (Figure 8.12F).

BOX 8.11 DEVELOPMENTAL STAGES OF LARVAL FISH

One of the most commonly used terminologies to describe the development of larval fish is based on that used by Ahlstrom and his co-workers (Moser et al. 1984; Neira et al. 1998; Leis and Carson Ewart 2000). The larval stage is defined as the development stage between hatching (or birth) and the attainment of full external meristic complements (that is, the number of fin rays and scales) and loss of specialisation for pelagic life. The larval stage is divided into preflexion, flexion and postflexion stages that are related to the development of the caudal fin and the corresponding flexion of the notochord. For example, two contrasting fish larvae show the relative size and stage of development.

herring

bream

Preflexion

6 mm

3 mm

Flexion

12 mm

6 mm

Postflexion

18 mm

10 mm

Figure 8.12 The flexion stages of some dominant families of fishes typically occurring in standard estuarine plankton collections: A-Engraulidae (anchovies), B-Clupeidae (herring, sprat), C-Hemiramphidae (garfishes), D-Atherinidae (hardyheads), E-Monacanthidae (leatherjackets), F-Paralichthyidae (flounders), G-Scorpaenidae (scorpionfishes), H-Platycephalidae (flatheads), I-Blenniidae (blennies), J-Sygnathidae (pipefishes, seahorses), K-Gobiidae (gobies), L-Sparidae (bream, tarwhine), M-Terapontidae (trumpeter), N-Sillaginidae (whiting), O-Gerreidae (silverbiddies), P-Chandidae/Ambassidae (glass perchlets), Q-Carangidae (trevallies), R-Monodactylidae (moonfish), S-Girellidae (blackfishes), T-Callionymidae (dragonets), U-Mugilidae (mullets). (Sources: Leis and Carson Ewart 2000; Neira et al. 1998).

Figure 8.12 The flexion stages of some dominant families of fishes typically occurring in standard estuarine plankton collections: A-Engraulidae (anchovies), B-Clupeidae (herring, sprat), C-Hemiramphidae (garfishes), D-Atherinidae (hardyheads), E-Monacanthidae (leatherjackets), F-Paralichthyidae (flounders), G-Scorpaenidae (scorpionfishes), H-Platycephalidae (flatheads), I-Blenniidae (blennies), J-Sygnathidae (pipefishes, seahorses), K-Gobiidae (gobies), L-Sparidae (bream, tarwhine), M-Terapontidae (trumpeter), N-Sillaginidae (whiting), O-Gerreidae (silverbiddies), P-Chandidae/Ambassidae (glass perchlets), Q-Carangidae (trevallies), R-Monodactylidae (moonfish), S-Girellidae (blackfishes), T-Callionymidae (dragonets), U-Mugilidae (mullets). (Sources: Leis and Carson Ewart 2000; Neira et al. 1998).

different depending on when they were caught. The inflation of the swim bladder is related to the diel vertical migration that larvae of many species undertake in estuaries.

Fish eggs are typically between 0.5 mm and 1.5 mm diameter, and are translucent with a clearly defined yolk, or embryo or oil globule(s) (invertebrate eggs are often dark and <0.5 mm diameter). Compared with larvae, there has been very little work undertaken on the identification of fish eggs. The characters that can be used to identify eggs are the egg size and shape, number, position and pigmentation of oil globules, the degree of yolk segmentation, chorion morphology, perivitiline space and embryonic characteristics (for example, Ahlstrom and Moser 1980).

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