Distribution, Biology and Management

The witch flounder, Glyptocephalus cynoglossus, is a demersal flatfish distributed on both sides of the North Atlantic. In the Northwest Atlantic, the species ranges from Labrador southward to Virginia, and is closely associated with mud or sand-mud bottom. In US waters, witch flounder are common throughout the Gulf of Maine and in deeper areas on and adjacent to Georges Bank and along the shelf edge as far south as Cape Hatteras. Witch flounder are assessed as a unit stock (Figure 10.1).

Witch flounder appear to be sedentary, preferring moderately deep areas; few fish are taken shallower than 27 m (15 fathoms) and most are caught between 110 and 275 m (60 and 150 fathoms). Witch flounder diet consists mostly of polychaete worms. Witch flounder attain lengths up to 78 cm (31 in.) and weights of approximately 2 kg (4.5 lb) (Bigelow and Schroeder 1953), but are slow-growing, late-maturing and can live as old as 30 years. Female witch flounder reach maturity between ages 5 and 6; spawning occurs in late spring and summer. The larval period is relatively long, between 6 to 12 months (Cargnellis et al. 1999).

Average fishing mortality (ages 8-9, unweighted) fluctuated between 0.26 and 1.12 between 1982 and 1996, but afterward declined to a record-low of 0.20 in 2004 (Figure 10.6 [Fig 10.6 Data]). Spawning stock biomass declined steadily from nearly 17,000 mt in 1982 to 3,900 mt in 1996, but has since increased to over 21,000 mt in 2004 (Figure 10.7 [Fig 10.7 Data]). Since 1982, recruitment of age 3 witch flounder has ranged from approximately 3 million fish (1984 year class) to 45 million fish (the 1997 year class), with arithmetic mean recruitment of 15.5 million fish (Figure 10.7 [Fig 10.7 Data]). The above year classes in the late 1990s have been followed by below-average year classes in early 2000s.

Biological Reference Points

Yield and spawning stock biomass biological reference points (Figure 10.8 [Fig 10.8 Data]) were last calculated in the 2003 assessment (Wigley et al. 2003) and reported in the 2005 GARM assessment (NEFSC 2005; Wigley and Col, 2005). These are given in Table 10.2.

The relationship between spawning stock biomass and recruitment for witch flounder over the period covering the 1982-2002 year classes is illustrated in Figure 10.9 [Fig 10.9 Data]. The stock-recruitment trajectory indicates the position of the most recent levels of SSB and recruitment on the far-right side of the plot, and illustrates the level of the 2002 year class that was produced from the 1999 spawning stock biomass. The solid horizontal line indicates arithmetic mean recruitment over the same period. Witch flounder exhibit a negative stock-recruitment relationship where low recruitment is associated with high stock biomass. This atypical relationship is seen in other flatfish stocks. Survival ratios, recruits per unit of spawning biomass (Figure 10.10 [Fig 10.10 Data]) illustrate the relatively low survival of recent year classes.

MSY-based reference points were updated in the 2003 assessment (Wigley et al. 2003), using the same method as the Working Group on Re-Evaluation Biological Reference Points for New England Groundfish (NEFSC 2002) which utilized yield and spawning stock biomass per recruit analyses and the arithmetic mean of age 3 recruitment. These updated reference points are given in Table 10.2.

Summary