Seasonal distribution, age, growth, and reproductive biology of marbled sole Pleuronectes yokohamae in Tokyo Bay, Japan

ABSTRACT:   The seasonal distribution, age, growth and reproductive biology of marbled sole Pleuronectes yokohamae (Günther) from Tokyo Bay, Japan were studied. A clear seasonal distribution trend was observed, with P. yokohamae limited to the south of the bay in summer and expanding almost throughout the bay in other seasons. The formation and dynamics of hypoxic areas in the bay limited the northward distribution of P. yokohamae in summer. Age was determined by counting growth rings on otoliths; maximum ages were found to be 5 years for males and 6 years for females. The mean back-calculated length for females at each estimated age was greater than that for males. Growth of males and females was shown by the von Bertalanffy growth equations. Reproductive biology was studied on the basis of gonadosomatic indices and histological sections of gonads. The spawning season lasted from November to March, peaking in December and January. Sixty per cent of males at 1 year and all males ≥ 2 years had mature testes, whereas 6.7% of females at 1 year, 58.3% of females at 2 years, and all females ≥ 3 years had attained maturity.     

Variation in the distribution of yellowfin sole Limanda aspera larvae in warm and cold years in the eastern Bering Sea

Multiyear periods of relatively cold temperatures (2007–2013) and warm temperatures (2001–2005 and 2014–2018) altered the eastern Bering Sea ecosystem, affecting ocean currents and wind patterns, plankton community, and spatial distribution of fishes. Yellowfin sole Limanda aspera larvae were collected from the inner domain (≤50 m depth) of the eastern Bering Sea among four warm years (2002, 2004, 2005, 2016), an average year (2006), and three cold years (2007, 2010, 2012). Spatial distribution and density of larvae among those years was analyzed using generalized additive models that included timing of sea-ice retreat, areal coverage of water ≤0°C, and water temperature as covariates. Analyses indicated a combination of temperature effects on the location and timing of spawning, and on egg and larval survival, may explain the variation in larval density and distribution among years. During warm years, higher density and wider spatial distribution of larvae may be due to earlier spawning, an expansion of the spawning area, and higher egg and larvae survival due to favorable temperatures. Larval distribution contracted shoreward, and density was lower during cold conditions and was likely due to fish spawning closer to shore to remain in preferred temperatures, later spawning, and increased mortality. Predicted drift trajectories from spawning areas showed that larvae would reach nursery grounds in most years. Years when the drift period was longer than the pelagic phase of the larvae occurred during both warm and cold conditions indicating that settlement outside of nursery areas could happen during either temperature condition.     

Comparison of the nutritional transition date distributions of marbled sole larvae and juveniles in Hakodate Bay, Hokkaido

To examine survival processes in marbled sole Pseudopleuronectes yokohamae larvae, nutritional transition date distributions (NTDDs) were compared between larvae and large juveniles (LJs) in Hakodate Bay from 2001 to 2003. NTDDs were used instead of hatch date distributions because the onset of increment formation coincides with the timing of the transition from endogenous to exogenous nutritional sources for marbled sole. LJs were defined as being large enough to be safe from predation by the sand shrimp Crangon uritai. In 2001 and 2002, the ranges of the NTDDs of LJs largely overlapped with those of the larvae. However, in 2003, the early cohort (those whose otolith rings formed before 17 March) disappeared from the NTDDs during and after April, and the NTDDs of LJs were heavily biased toward a later period compared to those of the larvae. In 2003, Oyashio coastal water (<3°C and ≤33.0 salinity) unexpectedly intruded into the bay in late March and lowered the water temperature from 6.3°C on 6 March to 4.1°C on 25 March. One possible cause of mortality in the early cohort of 2003 may be feeding failure caused by the extremely low water temperature.     

Patterns in the spawning of cod (Gadus morhua L.), sole (Solea solea L.) and plaice (Pleuronectes platessa L.) in the Irish Sea as determined by generalized additive modelling

Eleven ichthyoplankton cruises were undertaken covering most of the Irish Sea during the period February to June, 1995. To identify spawning localities and investigate temporal trends in egg production, the data on stage 1 A egg distributions of cod ( Gadus morhua ), plaice ( Pleuronectes platessa ) and sole ( Solea solea ) have been modelled using generalized additive models (GAMs). A two-stage approach was adopted where presence/absence was firstly modelled as a binary process and a GAM surface subsequently fitted to egg production (conditional on presence). We demonstrate that this approach can be used to model egg production both in space and in time. The spawning sites for cod, plaice and sole in the Irish Sea were defined in terms of the probability of egg occurrence. For cod, we demonstrate that by integrating under predicted egg production surfaces, a cumulative production curve can be generated and used to define percentiles of production and thus delimit the extent of the spawning season. However, for plaice and sole, the surveys did not fully cover the spawning season and the limitations that this imposes on GAM modelling of these data are discussed. Comparison of the spawning sites in 1995 with historical data suggests that the locations of cod, plaice and sole egg production in the Irish Sea have probably remained relatively constant over the last 30 years.