An evaluation of distribution, abundance, and phenotypes of early developmental stages of the common triple fin across a major life-history transition.

Many marine reef fishes have a bipartite life cycle, with reef-based adults that produce pelagic larvae. Pelagic larval development is characteristically risky; intrinsic and extrinsic factors often lead to high spatial and temporal variability in demographic rates. Pelagic larval development may also facilitate dispersal, and this — in combination with variable demographic rates during the larval stage — is likely to drive variation in local population replenishment (i.e., “recruitment”). Long-standing paradigms in benthic marine ecology rest on a belief that pelagic larvae are largely passive, and widely dispersed by ocean currents. However, recent work suggests that many reef organisms (particularly fishes) exhibit complex behavioural patterns, possess well-developed sensory systems, and have strong swimming abilities. These traits raise the possibility that dispersal may be the anomaly and near-shore larval retention (and potentially even reef-based larval development) may be relatively commonplace. If true, this could challenge fundamental ideas that underlie current management of many fisheries and alter our perceptions of the resilience of reef fishes to changing environmental conditions. At present, we continue to have a poor understanding of the early life histories of most reef organisms.

Here, I evaluate the early life history of the common triplefin (Forsterygion lapillum), a small-bodied reef fish found on rocky reefs throughout New Zealand. I sampled larvae during their development and transition back to the reef. I explored fine-scale spatio-temporal variation in abundance and individual traits, and otolith microchemistry, to infer some key developmental attributes and potential migratory patterns.

In chapter 2, I evaluated the near-shore distribution, abundance, and phenotypes of larval triplefin sampled by light traps deployed across the shelf (i.e., a gradient perpendicular to shore), and across depth. I found that late-stage larvae are concentrated in shallow surface waters, very close to the reef. Larval phenotype (e.g., body condition, and other metrics of body size) varied among sampling locations. Immediately above the reef, larvae sampled near the sea surface were in better condition than larvae sampled at depth. At offshore locations, this pattern was reversed. Spatial variation in abundance, condition and morphological traits of larval fish sampled across the shelf suggest one of two likely possibilities: (1) larval fish are highly mobile and may display an onshore movement pattern, forming shoals near the reef, which persist for a period of time prior to settlement, or (2) dispersal is relatively limited, and condition-dependent (i.e., many larvae of a wide range of sizes remain close to the reef, and only those in poor physiological condition are advected offshore).

In chapter 3, I used otolith microstructure to reconstruct age and growth patterns of these same larvae. I evaluated spatial variation in larval age and growth, in an attempt to differentiate between the two alternative hypotheses posed in the previous chapter. I found that larvae sampled above the reef were ~5d older than larvae sampled offshore, consistent with a hypothesis of ontogenetic onshore migration. Larvae sampled above the reef also had reduced daily growth rates relative to larvae sampled offshore. Finally, the ages of larvae sampled above the reef were substantially less than the reported settlement age and suggest that these larvae may be shoaling on the reef for up to two weeks prior to their eventual settlement.

In chapter 4, I developed and tested a method to infer movement patterns of larval fish based on the environmental fingerprints contained within their otoliths. Specifically, I reared larvae in water collected from different locations (e.g., offshore, near-shore, harbour, etc) and maintained at two different temperatures. I used laser ablation inductively coupled plasma mass spectroscopy (LA-ICPMS) to quantify the resulting concentrations of a set of elements within larval otoliths. I used multivariate statistical approaches to evaluate the ability to chemical signatures within otoliths discriminate among larvae that developed in different water masses. These approaches had some success in differentiating between individuals that developed offshore versus in nearshore/harbour water. In chapter 5, I again used LA-ICPMS to quantify the concentrations of a set of elements within otoliths of larvae sampled above reefs and offshore. I applied the multivariate statistical approaches that I developed in the previous chapter and used them to evaluate variation in chemical signatures recorded within (1) the core of the otolith (i.e., a putative natal signature), and (2) across the growing axis of the otolith (i.e., putative developmental histories). I did not detect significant variation in otolith core signatures, but patterns of variation in otolith microchemistry across the growth axis suggest that larvae collected offshore had significantly different developmental histories than larvae collected above the reef. I also observed significant temporal variation in otolith microchemistry.

Collectively, my results indicate a complex set of processes that may occur in the lead-up to settlement. Dispersal may be more limited that is widely assumed. Larvae of the common triplefin may move onshore and form shoals for several weeks prior to settlement. These movements may depend upon larval traits, and in particular, on body condition.