Alternative reproductive tactics and inverse size-assortment in a high-density fish spawning aggregation

Abstract

BackgroundAt high densities, terrestrial and marine species often employ alternate reproductive tactics (ARTs) to maximize reproductive benefits. We describe ARTs in a high-density and unfished spawning aggregation of the squaretail grouper (Plectropomus areolatus) in Lakshadweep, India.ResultsAs previously reported for this species, territorial males engage in pair-courtship, which is associated with a pair-spawning tactic. Here, we document a previously unreported school-courtship tactic; where territorial males court multiple females in mid-water schools, which appears to culminate in a unique ‘school-spawning’ tactic. Courtship tactics were conditional on body size, local mate density and habitat, likely associated with changing trade-offs between potential mating opportunities and intra-sexual competition. Counter-intuitively, the aggregation showed a habitat-specific inverse size-assortment: large males courted small females on the reef slope while small males courted equal-sized or larger females on the shelf. These patterns remained stable across two years of observation at high, unfished densities.ConclusionsThese unique density-dependent behaviours may disappear from this aggregation as overall densities decline due to increasing commercial fishing pressure, with potentially large consequences for demographics and fitness.Electronic supplementary materialThe online version of this article (doi:10.1186/s12898-017-0120-5) contains supplementary material, which is available to authorized users.

Hypothesizing novel mating behaviours in the squaretail grouper based on direct behavioural observations

“…However, this gives the video more importance than it deserves. The video was captured by a remote underwater camera, when we were NOT in the water and at no point in our paper do we present the video as conclusive evidence but merely as supporting what we observed in situ.The two incidents of 'school spawning' have been described as a sequence of events, culminating in gamete release in our manuscript (Karkarey et al 2017). These were observed directly by us in the water, at a distance of less than 5m from the female school.…”

“…At peak densities, we recorded an average of 72.08 ± 27.46 fish per 1000 m 3 (200m 2 × 5m depth) across the spawning site. Densities were more than 4 times higher on the slope of the spawning site, where we documented the alternative school spawning tactic (Karkarey et al 2017). Our paper explores how these observations might add to our understanding of the maintenance of alternative reproductive tactics (ARTs).…”

“…More fundamentally, the critique raises questions about the weight of evidence required to legitimately document unusual behaviours that may occur only at the extremes of population density, given their rarity. While we respond here to the principal objections raised in the critique, we encourage readers to read through our original manuscript, Karkarey et al (2017), where we have reported our observations with considerable detail and clarity.…”

“…In a manuscript published in the journal BMC Ecology (Karkarey et al 2017), we reported a rare set of observations of mating behaviours of the squaretail grouper (Plectropomus areolatus) at a spawning aggregation in the Lakshadweep Archipelago before the population was fished. The densities of groupers we recorded at the aggregation were among the highest recorded anywhere, and our detailed observations over two years confirmed that, at these high densities, this species shows two distinct courtship tactics -a pair courtship (also recorded elsewhere across the range of this species.…”

“…The authors of the critique call into question our observations and our broader understanding of grouper mating systems, although we have specifically acknowledged the diversity of mating strategies of groupers with relevant citations (Karkarey et al 2017, Page 2). As we clearly explain in our paper, the behaviour we report may be associated with particularly high density aggregations, which as we report, is extremely rare given the strong fishing pressures experienced by these groupers globally (Sadovy de Mitcheson et al 2013).…”