Spatiotemporal Comparisons of Young-of-the-Year Weakfish in Delaware Bay and Tidal Tributaries

1. Spatiotemporal Comparisons of Density, Growth and Production of Young-of-the-Year Weakfish in Delaware Bay and Major Tidal Tributaries Timothy E. Targett & Brian P. Boutin University of Delaware College of Marine and Earth Studies Lewes, DE 19958

2. Weakfish (Cynoscion regalis) Background • Estuaries serve as important seasonal nursery habitats for many ecologically and economically important species • Important to be able to determine/assess habitats in the ‘mosaic’ whose production per unit area is disproportionately greater than surrounding habitats • Higher densities, growth and survival of juveniles (YOY) • Habitats providing suitable physicochemical conditions, abundant prey and low predation risk that are thus important to fisheries (on both per unit area and total area bases)

3. Background The Sustainable Fisheries Act of 1996 (Public Law 104-279) Mandated the description, identification, and preservation of Essential Fish Habitat (EFH) • “Those waters or substrates necessary for spawning, breeding, feeding or growth to maturity”

4. Background • Four levels of information of increasing utility to designate EFH • Level 1: presence/absence • Level 2: habitat-specific densities • Level 3: habitat-specific growth, survival, and reproduction • Level 4: habitat-specific fish production • Data is often lacking for levels 3 and 4

5. The Delaware Estuary Variety of available nursery habitats for young fishes Tidal Tributaries Open Bay Spatial and temporal gradients of physicochemical and biological attributes Temperature range of 0-30+OC Salinity range of 0-30‰ Variable DO and nutrient levels Seasonal phytoplankton and vegetative marsh production Seasonal fish production Potential spatial patterns in growth and production as well NJ DE

6. Weakfish (Cynoscion regalis) Important seasonal component of fish assemblage Ecologically and economically important species Delaware State Fish! Spawning activity in spring Peak in late May and early June Multiple cohorts (usually two) Juveniles begin recruiting to nurseries in the early summer Return to coastal ocean in the fall

7. Three main studies on juvenile weakfish: Paperno et al. 2000, Grecay and Targett 1996, Lankford and Targett 1994 Abundance highest in the upper estuary Heavy recruitment in areas <20‰ Growth and survival: Upper estuary Lower growth rates, condition and gut fullness [Higher survival] Mid and Lower estuary Higher growth rates, condition and gut fullness Seasonal dynamics NJ Delaware Bay DE

8. Two main studies have sampled juvenile weakfish (NJ side) Able et al. (2001) found highest abundances in upper estuary tributaries Most abundant species in the upper estuary Nemerson and Able (2004) Upper and lower bay tributaries were similar in abundance in first year of sampling Lower bay tributaries had higher abundances in the second year of sampling Delaware Bay Tidal Tributaries

9. Focus has been on the Bay proper No studies have compared these two habitats in terms of habitat quality and nursery function for young weakfish Opportunity to build upon what is known Compare densities, growth and production Using a new technique (RNA:DNA) to examine short-term growth rates and production NJ DE

10. Objectives • Compare the spatial and temporal patterns in habitat-specific density, growth and production of YOY weakfish within and among six tidal tributaries of Delaware Bay and the adjacent open bay areas • Determine areas of highest density, growth and production of YOY weakfish and speculate on the most productive habitats in order to better define EFH for YOY weakfish

11. Methods • Study Sites • 6 Tidal Tributaries • Blackbird Creek (B), Simons (S), St. Jones (SJ), Murderkill (MK), Mispillion (M) and Broadkill (BK) Rivers • Lower, middle and upper stations • ~3km apart • 4 Bay Stations • Outside the mouths of each river (except Blackbird Creek) • St. Jones and Murderkill Bay station is the same

12. NJ Blackbird Creek (B) PA NJ Simons River (S) DE St. Jones River (SJ) DE Delaware Bay Murderkill River (MK) Mispillion River (M) Broadkill River (BK)

13. Methods • Field Sampling • Biweekly • July through early October • Replicate 150 m tows • 3 m otter trawl (38 mm stretch mesh net, 25 mm stretch mesh cod end, 5 mm square mesh cod end liner) • All sampling within 2 h of high tide • Temperature, salinity and DO measurements at each station • Up to 15 fish per site per day were frozen on dry ice for short-term growth rate (RNA:DNA) analysis

14. Methods • YOY weakfish habitat-specific densities • Numerical densities calculated as number of weakfish caught per tow at a specific station on a specific day by the area swept (382.5 m2 for riverine stations, varied for Bay stations) • Biomass densities calculated as above using station-specific YOY weakfish mass per tow

15. Methods • Weakfish in situ growth rate assessment • Requires a method that can estimate growth over very short time periods to afford small scale temporal and spatial comparisons • Essentially “instantaneous” growth rates • RNA:DNA / growth rate relationship

16. Low ration Medium ration No food Ad libitum 20°C 23°C 27°C 30°C Methods RNA:DNA Growth Rate Food type: Mysis relicta N = 5 fish per treatment

17. Predictive Relationship • From Stierhoff, Targett & Power (submitted)

18. Temporal Sensitivity • RNA:DNA of juvenile weakfish responds in very short time periods (~48 h) to changes in feeding and growth conditions

19. HABITAT-SPECIFIC GROWTH AND PRODUCTION OF YOY WEAKFISH P= Gs*B Gs is an individual’s short-term growth rate calculated from the RNA:DNA to growth rate relationship B is the station-specific mean daily biomass density  Snapshots of growth  Snapshots of production

20. Results • Multitude of Comparisons • Spatial • Bay, Tributary, Stations within Tributaries • Temporal • Sampling Weeks - Seasonality • Focusing on the major findings of the first year’s data (2005)

21. Seasonality Numerical Density Biomass Density Production • Seasonal decline in numerical and biomass densities • Seasonal decline in growth rate • Large drop in August • High temperature effects on SGR and food availability • Seasonal decline in production • Low growth in August and declining biomasses causing production levels to plummet • Slight rebound in September due to recovery of growth but production constrained by declining biomass Growth Rate

22. Overall Spatial Trends Biomass Density Numerical Density Production Growth Rate • Significantly higher numerical and biomass densities in adjacent Bay • Significantly higher growth rate in adjacent Bay • Some indication of higher production levels in adjacent Bay

23. Bay vs Tributaries Numerical Density Biomass Density Tributary Bay B Production Growth Rate BK • Numerical and biomass densities generally higher in the Bay than in the adjacent tributary locations • Growth rate consistently higher in Mispillion (M) and Simons (S) Bay locations • Production highest within the middle portions of the estuary • Lower Murderkill and Mispillion Rivers • Occasional coincident very high biomass and high growth rates in these rivers  high overall production values  Suggests absence of density-dependent growth

24. Bay vs Tributaries Numerical Density Biomass Density Tributary Bay B Production Growth Rate BK • Production was also consistently high in the bay sites off the Mispillion and Simons Rivers, due to high biomass at both locations and high growth rates, particularly at the Mispillion Bay site

25. Conclusions To-Date • First year results suggest the most productive nursery areas for young weakfish in the Delaware Estuary are in the lower portions of mid-bay tidal tributaries and the adjacent open bay • Incidents of very high production within tributary habitats • Highest biomass densities frequently coincide with highest growth rates  high overall production values • No evidence of density-dependent growth • The approach used here has promise for examining patterns and processes underlying (facilitating or limiting) productivity at upper trophic levels in the Delaware Estuary

26. Acknowledgements • National Estuarine Research Reserve • Graduate Research Fellowship • Delaware NERR • Bob Scarborough, Mike Mensinger • T. Targett Lab • Graduate Students • Mike Rhode, Ben Ciotti, Damian Brady, Danielle Tuzzolino • Technicians • Scott Baker, Nate Campbell Catherine House, Taylor Unger