Patterns in the spawning of cod (Gadus morhuaL.), sole (Solea soleaL.) and plaice (Pleuronectes platessaL.) in the Irish Sea as determined by generalized additive modelling

Abstract

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.

Identification of marine fish egg predators using molecular probes

“…Cruises were conducted in 2008 and 2009 on a well-described plaice spawning ground (Fox et al 2000a in the eastern Irish Sea (Fig. 1).…”

“…Euphausiids have also been reported to consume fish eggs and larvae (Bailey et al 1993) but they were not abundant in the egg patch investigated. Euphausiids are much more common in the deeper parts of the western Irish Sea (Mauchline 1984) where they could be more important as predators of fish eggs, since these areas are also adjacent to fish spawning grounds (Fox et al 2000a).…”

Determination of species-specific spawning distributions of commercial finfish in the Irish Sea using a biochemical protein-based method

“…Spawning has been extensively described and quantified for Irish Sea stocks of cod Gadus morhua, haddock Melanogrammus aeglefinus and plaice Pleuronectus platessa (Simpson 1959, Nichols et al 1993, Anonymous 1997, Ellis & Nash 1997, Fox et al 1997, 2000, Nash & Geffen 1999, Armstrong et al 2001. The accuracy of these estimates remains contentious, however, due to potential misidentification of eggs that are spawned simultaneously in spring in inshore coastal waters of the eastern and western Irish Sea (Anonymous 1997, Fox et al 2000, Armstrong et al 2001. Conventionally, marine fish eggs are identified by visual characteristics such as the presence, number and size of oil globules, size of the perivitelline space, yolk homogeneity and embryo pigmentation (Russell 1976).…”

“…Usually, the relative abundance of these late-stage eggs or early-stage larvae is used to estimate the distribution and abundance of newly spawned (Stage IA) eggs (Page et al 1997, Bradbury et al 2000. Species that could be mistaken for the target species and are rare as larvae are often assumed to be absent from the plankton when in early developmental stages (Anonymous 1997, Fox et al 2000, Armstrong et al 2001. Later developmental stages are likely to have drifted from spawning grounds over time, introducing uncertainty to distribution estimates.…”

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“…They found that the water depth is a determinant factor in habitat use by small and large crayfish. Fox, Brien, Collas, and Nash (2000) modeled egg production patterns and egg density of three marine fish species in the Irish Sea using environmental variables. Location, depth, time, depthintegrated sea-temperature and salinity were incorporated into the presence/absence and egg production models.…”