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Amphidromous gobies are circumtropically distributed in mountainous coasts typical of volcanic islands. More than nine goby species recruit seasonally from the Pacific coasts to the Hsuikuluan River estuary eastern Taiwan from the spring to the summer. Using two seasons of field observations and sample collections, we explore the recruitment of amphidromous gobies from the Pacific Ocean into the Hsuikuluan River and their upstream migratory behaviors from the estuary to the lower reaches of the river. A metamorphosis check in the otolith which corresponds to the drop of Sr/Ca ratios from higher > 4 × 10−3 to lower levels

The composition, structure and seasonality of ichthyoplankton in the surf zone adjacent to the mouth of the intermittently open East Kleinemonde Estuary (33°32′S, 27°03′E) were investigated over a period of 2 years. Altogether 451 fishes, representing at least 21 taxa from 14 families, were collected. The assemblage was dominated by postflexion larvae of euryhaline marine species that are dependent on estuaries as nursery areas. The sparid Rhabdosargus holubi was the most abundant taxon and constituted more than 77% of the total catch. A distinct modal size class was identified forR. holubi , while the mean individual size of this and other abundant taxa was comparable to the observed recruitment size range reported from a wide variety of estuarine nursery habitats in southern Africa. Periodic regression analyses revealed significant peaks in abundance of larval R. holubi during late winter (August), at down and dusk, at new and full moon (spring tides), and on the flood stage of the tidal cycle. Evidence for estuarine immigration during marine overwash events (surging rough seas that enter the estuary) is provided by (1) the stranding of postflexion larvae in the region of the closed estuary mouth following these events, and (2) back extrapolation from length modes within the estuary to coincide with such an event. The advantages and disadvantages of such a recruitment strategy are discussed.

Timing of life-history stages and environmental conditions is key to recruitment success. We examine the seasonal pattern in access between the surf zone and an estuary, and the implications for recruitment success and process in fish that spawn at sea but spend their juvenile phase in estuaries. About 70% of South African estuaries are closed by barrier sand bars that open intermittently; an alternative but brief access opportunity is marine overwash (overtopping), when the surf zone extends over the bar to contact estuarine waters. Larval fish have limited ability to wait for an access opportunity (overtopping or opening event), so timing of settlement is important with respect to the seasonal distribution of waiting times for access opportunities. Periodic regression on daily observations (1993–1999) at the East Kleinemonde Estuary profiled the seasonal variation in expected waiting time. The data set is dominated by overwash events. Waiting time (for all events) is significantly related to both the first and second harmonics of season, and tends to be longest in December–January, and shortest in April–May. If the analysis is restricted to openings alone waiting times are shortest near November, and longest from February to September. The seasonal variation shown has implications for recruitment processes, population sizes, and productivity in estuaries.

Even with constant production (N0) and constant time-specific mortality (Z, daily), recruitment can vary greatly. A new variable-lag phenomenon modelled here generates temporal variation in recruitment from temporal shifts in age-at-recruitment (AAR; here synonymous with stage duration). The phenomenon is due to two components, one being analogous to the Doppler effect and the other an interaction of dAAR/dt with Z, and is distinct from those identified in previous contributions not focusing on temporal structure. Recruitment can be predicted from N0, Z, and AAR, using either numerical simulation or, in many cases, the modified Doppler equation given here. Seasonal recruitment cycles predicted for a sinusoidal AAR function fitted to natural data, with constant larval production and constant Z (varied among, not within, model runs), were strongly peaked with a difference of 20% between maximum and minimum recruitments at Z = 0, to a difference of 2000% at Z approx 0.2. The model gives indirect access to parameters difficult to measure in the field: if temporal variation is adequately described in two of the three input variables (N0, Z, or AAR), differences (residuals) between predicted and observed recruitment can indicate an unaccounted pattern in the unknown.

We report on an 18-month field study of temporal pattern and interrelationships of size- and age-at-recruitment (to fresh waters) for two diadromous hill-stream gobies in Dominica, West Indies (15°30′N, 61°25′W). Recruitment is panseasonal in Sicydium punctatum and Sicydium antillarum and supports a directed fishery similar to other goby fry fisheries. We found both size and age of recruits to vary seasonally, but the relationship of size to age is complex. Although positively related within lunar-monthly recruiting cohorts, the overall relationship of size and age was poor because seasonal variation in size is out of phase with age. Among recruiting cohorts, mean (or predicted) size and age are negatively correlated. Recruitment (fishery yield) also varies seasonally, in phase with size and out of phase with age. Peak fishery yields coincide with the youngest, but largest, recruits; low yields coincide with the reverse, implicating growth rate variations in fishery yield. This is the first such report of systematic seasonal variation in age-at-recruitment. Given these findings, size- and age-at-recruitment (here similar to settlement or metamorphosis) should not be assumed invariant, even for tropical fishes.