of primary productivity, e.g. chlorophyll a (Deegan 1990, Govoni 1997, Houde et al. 2016, Salvador & Muelbert 2019).
3.6.5 Morone saxatilis (Moronidae)
The anadromous Morone saxatilis spawns near the salt–fresh interface in estuaries on the east and west coasts of North America, although the west coast populations were introduced. Daily cohorts are produced during a two‐month spawning period (Rutherford & Houde 1995). Year‐class strength is coarsely set when larvae are only 8 mm in length and is determined by mortality and growth rates operating during the egg and larval stages (Secor & Houde 1995, Rutherford et al. 1997). The highest mortalities occur in egg, yolk‐sac and early‐feeding larval stages when >99% cumulative mortality typically occurs in spawning tributaries of Chesapeake Bay (Rutherford & Houde 1995, Secor & Houde 1995) (Table 3.2). Weight‐specific growth and mortality rates of larvae are high and variable and are responsive to local environmental factors (Houde 1996), particularly those near the estuarine salt front and turbidity maximum zone (North & Houde 2001, 2006). Amongst the interacting factors that control mortality and growth of larvae are precipitation and freshwater flows, prey abundances, temperature, conductivity (salinity) and pH (Uphoff 1989, Secor & Houde 1995, Rutherford et al. 1997, Limburg & Pace 1999, Kimmerer et al. 2001, Secor et al. 2017). In estuaries with the most extensive spawning areas, for example the tidal Hudson River system (USA), biotic variables, such as prey abundance, may play a more important role than hydrography in determining survival and growth (Limburg & Pace 1999). However, in the large and dynamic St Lawrence River system (Canada), the estuarine turbidity maximum region is a key feature associated with successful feeding and production of young (Vanalderweireldt et al. 2019a, 2019b).
Table 3.2 Stage‐specific mortality rates and abundances at the onset of stage for Morone saxatilis in the Patuxent River sub‐estuary of Chesapeake Bay in 1991. YSL = yolk‐sac larvae; FFL = first‐feeding larvae; PFL = post‐finfold larvae; t = days in stage; N stg = abundance calculated at youngest age for each stage: Z t = daily mortality rate (d−1); Z stg = cumulative mortality per stage (from Secor & Houde 1995).
| Stage | N stg | t | Z t | Z stg | Stage (%) |
|---|---|---|---|---|---|
| Egg‐YSL | 6.46 × 108 | 2.2 | 0.09 | 0.20 | 18.1 |
| YSL‐FFL | 5.43 × 108 | 6 | 0.97 | 5.80 | 99.7 |
| FFL‐PFL | 1.64 × 106 | 20 | 0.15 | 2.95 | 94.8 |
| PFL | 0.09 × 106 | 25 | 0.07 | 1.81 | 83.6 |
Mortality rates and cumulative mortalities of Morone saxatilis larvae are dependent on temperature. Larval cohorts experiencing lowest mortality develop at intermediate temperatures of 17–19 °C that are near the median in the seasonal range (Secor & Houde 1995, Rutherford et al. 1997). High mortalities are associated with slow growth at lower temperatures, while increased predation may cause high mortality at higher temperatures (Secor & Houde 1995). Weather events, for example drops in temperature to lethal levels (12 °C), often combined with wind events that disrupt the retentive salt front and estuarine turbidity maximum (ETM), can generate high, cohort‐specific mortalities (Secor et al. 1995, Rutherford et al. 1997).
Growth and survival of Morone saxatilis larvae are primarily density independent (Kimmerer et al. 2000, Martino & Houde 2012) and responsive to the sufficiency of zooplankton prey resources and the timing of prey availability in nursery areas (i.e. supporting the match‐mismatch hypothesis; Cushing 1990). Timing of production of two key prey, the copepod Eurytemora carolleeae (= affinis) and a cladoceran Bosmina sp., is recognised as important for production of M. saxatilis larvae (Limburg & Pace 1999, Campfield & Houde 2011, Vanalderweireldt et al. 2019a).
The salt front and estuarine turbidity maximum regions of estuaries, which are important to support growth and survival of Morone saxatilis larvae, are best developed under conditions of moderate‐to‐high freshwater discharge that favour retention of eggs and larvae (Kimmerer et al. 2001, North & Houde 2001, 2003, Martino & Houde 2010). The positive relationship between spring or spring‐summer, freshwater discharge and survival of larvae, and subsequent juvenile production, is documented for Chesapeake Bay (Martino and Houde 2004) (Figure 3.21). Retention and up‐estuary transport of larvae near the salt front and ETM were demonstrated in larval mark‐recapture experiments (Secor et al. 1995, 2017) in which millions of chemically marked, hatchery‐produced larvae were released into two Chesapeake Bay tidal tributaries.
Figure 3.21 Relationship between freshwater flow in spring and summer months and young‐of‐the‐year juvenile recruitment levels (mean catch per seine haul) for Morone saxatilis in Chesapeake Bay. Years 2001 (average flow), 2002 (low flow) and 2003 (high flow) are indicated
(from Houde (2016, his Figure 3.23), modified from figure 5 in Martino & Houde (2004).
Data from Maryland Department of Natural Resources (https://dnr.maryland.gov/fisheries/pages/striped‐bass/juvenile‐index.aspx).
3.6.6 Gadidae and Clupeidae (Baltic Sea)
The Baltic Sea is a large enclosed, saline water body that supports reproduction by marine and freshwater fishes. For the gadid Gadus morhua, a typically marine species, the ambient salinity in the Baltic Sea is insufficient to maintain floating eggs and they sink to a depth of neutral buoyancy such that peak abundance occurs near the halocline in the Bornholm Basin, with smaller numbers in the more saline deep layer (Westin & Nissling 1991, Nissling et al. 1994, MacKenzie et al. 1996, Wieland & Jarre‐Teichmann 1997). Larvae of G. morhua typically hatch within 15 days of spawning and migrate vertically through the halocline into the low‐salinity surface layers (30–40 m depths) to feed (Grønkjær & Wieland 1997, Grønkjær et al. 1997). Dispersal of G. morhua larvae is primarily resulting from wind‐driven circulation in the Baltic Sea (Voss et al. 1999). Wind stress results in Ekman transport within coastal jets along both coasts of the Bornholm Basin. Vertical distributions of the larvae indicate that drift in the Bornholm Basin mainly occurs in a compensating return flow below the Ekman
