Hydrobiologia DOI 10.1007/s10750-017-3182-7 MANGROVES IN CHANGING ENVIRONMENTS Fish communities and juvenile habitat associated with non-native Rhizophora mangle L. in Hawai‘i Stacia D. Goecke . Susan M. Carstenn Received: 16 November 2016 / Revised: 30 March 2017 / Accepted: 3 April 2017 Ó Springer International Publishing Switzerland 2017 Abstract There are many well-documented ecosys- and higher percent mangrove cover. Stream mouth tem services for which mangroves have received estuaries with mangroves are important juvenile protection globally; however, in Hawai‘i, where no habitats for the native K. xenura and M. cephalus species of mangroves are native, these services have and the non-native Osteomugil engeli. These species, not been thoroughly evaluated. Twelve permanently two of which are important in recreational and open stream mouth estuaries on O‘ahu were sampled subsistence fisheries, will be most likely affected by from September to October 2014. Abiotic factors and mangrove removal based on abundance and distribu- fish community data were correlated with percent tion in mangrove-dominated stream mouth estuaries. mangrove cover and the Landscape Development Intensity Index to examine potential relationships Keywords Landscape development intensity index Á among mangroves, humans, and fish communities. Kuhlia xenura Á Mugil cephalus Á Stream mouth The three most abundant species, of 20 species caught, estuaries Á Juvenile abundance Á Invasive species were Kuhlia xenura, Mugil cephalus, and Mul- loidichthys flavolineatus. Of these three native spe- cies, 99% of the individuals caught were juveniles indicating the overall importance of stream mouth Introduction estuaries as juvenile habitat. Non-metric multidimen- sional scaling analysis of fish community data showed In Hawai‘i, there are two types of estuaries: bays or that K. xenura, M. cephalus, and Lutjanus fulvus were saturated river valleys, and tidally influenced stream more abundant in shallower sites with lower salinity mouths (Kam et al., 1975), also referred to as stream mouth estuaries (SMEs). These SMEs may be peren- nial or permanently open, discharging into the ocean year-round, or intermittently open, discharging sea- Guest editors: K. W. Krauss, I. C. Feller, D. A. Friess, R. R. Lewis III / Causes and Consequences of Mangrove sonally, or only after heavy rain (Kam et al., 1975). Ecosystem Responses to an Ever-Changing Climate The stream mouth of permanently open SMEs is permanently connected to the ocean from the fresh- & S. D. Goecke ( ) Á S. M. Carstenn water source. Stream modifications and diversions Hawaii Pacific University, 45-045 Kamehameha Highway, Kaneohe, HI 96744, USA used for development and agriculture now prevent or e-mail: [email protected] minimize stream flow to the full downstream extent S. M. Carstenn and into many historical stream mouths. SMEs, in the e-mail: [email protected] context of this study, include embayment areas where 123 Hydrobiologia there is an open connection between freshwater southwestern Moloka‘i in 1902 and to He‘eia, O‘ahu sources and the ocean along the coastline; some of in 1922 (Allen, 1998). It is now well-established in these freshwater sources are natural (streams) and many coastal areas of Hawai‘i, including SMEs. All some are anthropogenic drainage. Mangroves are mangroves in Hawai‘i are non-native and R. mangle is often associated with SMEs. considered invasive. Land use/human impacts can alter environmental Mangroves influence physical and chemical prop- conditions, particularly along watersheds. Less devel- erties. Mangroves are highly productive and therefore oped areas with more vegetation cover have slower add more vegetation (stand structure and propagules) heat increases and smaller temperature fluctuations, and detritus (leaf litter, wood debris) to SMEs than whereas developed areas result in overall warmer native plants found in similar habitats in Hawai‘i temperatures and larger temperature fluctuations (Allen, 1998; Cox & Allen, 1999; Demopoulos et al., (Weng, 2001; Brasher, 2003; Weng et al., 2004). 2007). Decomposition of these materials may lower Surface temperatures increase with increased imper- dissolved oxygen and increase particulate and dis- vious surface (concrete/asphalt) due to reductions in solved organic matter. Mangrove vegetation can vegetation canopy cover and shading. Areas that are reduce water flow, velocity, and tidal drainage more developed often have increased stream and (Kitheka, 1997), decrease tidal currents (Furukawa groundwater diversion and impervious surfaces, lim- et al., 1997), and limit water exchange (Wu et al., iting the amount of freshwater reaching the SME. This 2001). Mangroves may cause complete blockages of decreases fluvial and groundwater influences, leaving the stream channel, further limiting water mixing the SMEs more affected by tidal influences, thereby within the estuary (Wu et al., 2001). In Hawai‘i, some increasing salinity. Streams, rivers, and run-off con- of the concerns regarding mangroves include drainage nect the entire watershed allowing resulting effects of impedance, poor aesthetic qualities, reduction of disturbances throughout the watersheds to accumulate habitat for native shorebirds, and damage to archeo- in the SMEs. SMEs and their inhabitants are thus logical sites (Allen, 1998; Rauzon & Drigot, 2002). especially vulnerable to disturbances, including The latter two have motivated efforts around the human impacts (e.g., land use/land cover alterations Hawaiian Islands, including Kaloko-Honoko¯hau and stream water diversion), and particularly impor- National Historic Park in Kona, Hawaii, Marine Corps tant subjects for research and coastal management. Base (MCBH) on the Makapu‘u peninsula, Ka¯ne‘ohe, Mangroves require tidal flushing (Lugo & Sneda- O‘ahu, and Pearl Harbor National Wildlife Refuge, ker, 1974; Schaeffer-Novelli et al., 1990; Harris et al., Pearl Harbor, O‘ahu, to remove the mangroves and 2010), yet sea-level rise, storm surges or rough water restore native vegetation. Proper planning and a conditions can damage, stress, or kill mangroves and thorough understanding of specific mangrove areas affect their distribution (Lugo & Snedaker, 1974; throughout Hawai‘i are essential to better inform Doyle et al., 1995; Gilman et al., 2007; Smith et al., management and minimize expense, effort, and 2009). Primary productivity of Rhizophora mangle L. potential disturbance when considering mangrove decreases with increased salinity (Lugo & Snedaker, removal in SMEs. 1974), but it is more tolerant of salt water than other In their native range, leaf litter and shading from species in Hawai‘i, e.g., Bruguiera sexangula (Lour.) mangrove canopy cover creates refuge for fish (Ellis & Poir. (Clough, 1992). Increased salinity decreases Bell, 2004). The extensive stand structure and organic proxies for photosynthetic rates (leaf stomatal con- sediment from mangroves provide nursery habitat for ductance and leaf chlorophyll fluorescence induction) fish and shellfish species targeted in recreational and in R. mangle suggesting an optimal salinity of less than commercial fisheries (Vidy, 2000). Juvenile fish 20 ppt for increased photosynthesis (Biber, 2006). dominated total catch in mangrove studies in Brazil, Optimal growth of a closely related species, R. West Africa, and Taiwan (Kuo et al., 1999; Vidy, mucronata, was at 50% seawater (16–18 ppt), with 2000; Krumme et al., 2004). Mangroves support fish decreased growth in salinity higher than 16–18 ppt populations by providing refuge and food, especially (Aziz & Khan, 2001). SMEs on O‘ahu are a balanced for juveniles (Nagelkerken & Faunce, 2008). A combination of calm, brackish water, ideal for R. nursery habitat has previously been defined as ‘‘any mangle. This species was first introduced to habitat that makes a greater than average contribution 123 Hydrobiologia to the recruitment of adults’’ (Beck et al., 2001). Intensity Index (LDI) is a weighted measure of Nursery function may be measured as the dominance potential disturbance to an ecosystem that is used to of species in juvenile stages (Nagelkerken et al., quantify a gradient of human impact based on land use 2000). Beck et al. (2001) states that any combination (Brown & Vivas, 2005). It was hypothesized that of four factors (density, growth, survival of juveniles, SMEs with higher percent mangrove cover would and movement into adult habitats) may be considered exhibit lower diversity of native species, species to assess contribution to adult recruitment and thereby richness and evenness, and higher overall biomass and identify nursery habitat. The definition of nursery relative abundance of individual fish, specifically habitat continues to evolve. Two papers published juveniles. It was also hypothesized that SMEs with after the current study was conducted, Nagelkerken higher LDI would exhibit lower diversity of native fish et al. (2015) and Sheaves et al. (2015), expand this list species, fish species richness and species evenness, to include assessment of connectivity and complexity and higher diversity, biomass, and relative abundance within potential nursery habitats as well as with of non-native fish species. adjacent coastal areas to determine nursery value. Analyses of correlations among percent mangrove Despite being well-documented as essential nursery cover, LDI, abiotic factors, and fish communities habitat in their native range worldwide, the function of assess the suitability of SMEs as nursery habitat, non-native mangroves specifically as