Monitoring and Conservation of Freshwater and Marine Fishes: Synopsis

1. Introduction

Globally, native migratory and resident fishes are declining from aquatic and terrestrial ecosystem degradation resulting from physicochemical habitat alteration, migration barriers, over-exploitation, hatchery supplementation, non-native species introductions, and the climate crisis [1]—all driven by human overpopulation and excessive energy and materials consumption [2]. Loss of diadromous fishes reduces marine-derived nutrients that are important for freshwater and floodplain biota [1,3], including riparian trees that protect freshwater ecosystems from land use [4]. The depletion of marine and freshwater fishes threaten natural-resource industries, human food supplies, and ecosystem processes [5,6]. Healthy aquatic ecosystems have diverse habitats that house a diversity of fish species, including various trophic, habitat, reproductive, and life-history guilds [1]. To better protect fish resources, which provide recreation and sustenance for millions of people, rigorous monitoring is important for assessing fish assemblage and population health and their limiting factors [1,7]. Therefore, this Special Issue focuses on ecological analyses based on large sample sizes over relatively large areas.

Recently, we reviewed the use of multimetric indices (MMIs) for assessing the ecosystem condition of aquatic and riparian ecosystems [1,8]. The former paper was stimulated by prior research indicating natural, longitudinal shifts in food webs and positive relationships between sample size and fish species richness. We concluded that insufficient and inconsistent sampling confounded anthropogenic impact analyses when too few fish are collected at sites or too few sites are sampled [1]. We were also concerned that MMIs are subject to ad hoc modifications of metrics, thus requiring calibration across regions [8,9,10,11]. Those calibrations highlight the need for more general MMI metrics [1], which we hope to better achieve with this Special Issue. Others have indicated the critical importance of rigor in determining reference conditions for making biological-impact assessments [8,10]. Again, several papers in this Special Issue lend credence to those concerns.

We volunteered to edit this Special Issue because of our concern for better mechanistic understanding of fish-environmental relationships globally to improve fish-assemblage monitoring. So, we encouraged submitting authors to examine aquatic conservation at multiple biotic and spatiotemporal extents for more-effective management and monitoring [12,13]. Notably, fishes suffer cumulative impacts that often complicate their recovery efforts [12,13,14], especially in the face of the climate crisis [5,6,15,16,17] and non-native fish invasions [1,4,16,17,18,19].

2. Synopsis for Special Issue

2.1. Assessments of Drought or the Climate Crisis

Several papers dealt with drought or the climate crisis, which are increasing problems for stream fishes globally. Pompeu et al. [20] demonstrated the importance of ichthyoplankton surveys for identifying critical habitats for riverine fishes in the face of dams and other impacts. This Brazilian study was undertaken during a severe drought year and indicated the essential need to use eDNA and frequent sampling to accurately determine spawning rivers and periodicities [20]. We expect that eDNA will become increasingly important for monitoring fish assemblages [21,22,23], especially for those species that are rarely encountered with traditional techniques [1,20].

Hamilton et al. [24] was a microbiologic study. Although fish diseases have often been incorporated into MMIs to assess anthropogenic impacts [1], these relationships need better development, especially considering that disease spread is exacerbated by non-native fishes [25,26] and the climate crisis [27]. Hamilton et al.’s [24] salmonid-oriented paper, which admirably used Inuit First Nation help for the research, suggests that lake whitefish may be maladapted in northern Canada to deal with the climate crisis. Such climate-crisis disease sensitivity needs further elucidation in both temperate and tropical regions [27], as do the synergistic effects of eutrophication and the climate crisis [1,28].

Robinson et al. [26] examined a water-limited, agricultural river basin housing both native and non-native fish species. The natives often showed reduced ranges more recently but not obvious spatially extensive presence/absence trends over the last two decades, partly because of three different lifespan classes (<3 y, 3–6 y, >6 y) that require different study durations. Likewise, 12 years of data were needed to reveal a statistically significant ecohydrological trend in a Washington, USA, stream [7]. Power analyses to reduce Type-II statistical errors of not finding real impacts are critical for aquatic bioassessments in the face of continuing development pressures [1,5,29]. Robinson et al. [26] also addressed the concept of ‘shifting baselines’ [7] that plague biomonitoring and power analysis [1] to enhance the general usefulness of their paper for climate crisis research. Australian fish assemblages are adapted to “boom vs. bust” climatic fluctuations via rapid dispersal thereafter, but climate-mitigation management is nevertheless needed to prevent fish kills. This should include both fish-passage and smaller-extent rehabilitation efforts [26]. Wetland-specific sampling is also needed given wetland losses that affect fishes [1,3,30]. Robinson et al. [26] should prove to be an important paper for climate crisis, wetland, and instream-flow fields.

Bergström et al. [31] examined climate-based behavioral evolution. They found that a Swedish population of Wels catfish (Silurus glanis) in a mesotrophic lake showed different adult-foraging behavior than this catfish did farther south in Europe, based on a mark-recapture study and comparative literature. For the Swedish lake, summer/fall activity included nocturnal, pelagic feeding, but settling near the bottom for diurnal resting. This contradicts the general tenet that zooplankton and forage fishes are less vulnerable to nocturnal predation in lake epilimnia, although lower trophic levels went unstudied there. In not preferring warmer waters, including their display of late-winter activity under ice, this catfish’s behavior differed from that of southern populations, which are dormant at such lower temperatures; prefer shallow, vegetated, sheltered bays; and differ in genetics, growth rates, and longevity. Moreover, habitat connectivity was important for successful lake feeding and creek reproduction (in late spring) in such northern, migrating populations, which have been long-isolated from their southern conspecifics [31]. Lake warming also was reported to decrease the length structure of northern pike (Esox lucius) in Lake Windermere (UK [32]). The short-duration, northern summers likely promoted such evolutionarily divergent seasonal behavior.

2.2. Assemblage Assessments Dealing with Non-Native or Stocked Species

In their spatially extensive mapping analysis to better achieve migratory-fish management, Kajee et al. [33] located native, endemic, threatened, and non-native species hotspots in South Africa in an important gap analysis study. Notably, non-native vs. threatened species records overlapped in over 50% of the area, as nearly half the threatened species records were outside protected areas and non-natives occurred in over a third of the protected areas [33]. Jelks et al. [34] also reported that habitat degradation and non-native fish were the major threats of at-risk North American fish species. Stream barriers were useful for excluding non-natives from the spawning habitats of native, migratory fishes in larger basins, which was also reviewed globally by Jones et al. [35].

Aparicio et al. [11] considered both native and non-native fishes in Spanish streams. Although non-native fishes typically degrade ecosystem processes and natural biodiversity [1,16], non-native fishes are often excluded from MMIs (but see [10,36]). Aparicio et al. [11] nicely included a separate metric for non-native pressure on native species and found that explicitly including non-native fish pressure provided a more comprehensive assessment of ecosystem health than did the European MMI without that metric.

Faro et al. [37] also addressed native and non-native fishes by holistically examining land use (especially agriculture), eutrophication, and hydromorphologic (e.g., dam-hydropeaking) criteria to classify Portuguese sites into four levels of human impacts. They emphasized the biophysical importance of intact riparian areas as native-fish habitats. Perhaps the most interesting is that they relied on just four fish-assemblage metrics (as percentages) in their MMI: native lithophils (for spawning), non-natives, migrants (via diadromy/potamodromy), and freshwater natives. The three metrics besides non-native fishes were associated with less-disturbed conditions, whereas non-natives were associated with more-stable flow regimes than native species, which preferred naturally varying flows [37]. Ruaro et al. [18] reported that fish MMI scores declined with increased abundance of non-native species in two Brazilian river basins. Faro et al. [37] found only partial support of better biotic conditions away from hydropeaking impacts; however, based on a large, Europe-wide database, Schinegger et al. [38] found that fishes were intolerant of hydrological stressors alone and when combined with morphological stressors. Dams are well-known for blocking migratory-fish access to upstream, downstream, and floodplain habitats, which also get degraded for landlocked fishes and their foods [1,3,4,5].

Wildhaber et al. [39] addressed dam impacts via a long-term database. They undertook a fish-habitat analysis for sicklefin chub (Macrhybopsis meeki) and sturgeon chub (M. gelida) and their piscine predators in the mainstem Missouri River, where hydropower production and channelization prevail. Some of the piscivores both predated on and competed with the chubs. The chubs were best caught by benthic trawling, but many netting and other methods were used for more-complete sampling intra- and interspecifically. The two cyprinids were subjected to habitat-occupancy modeling, including their use of both main- and off-channel habitats, and showed marked spawning-flow relationships, suggesting flow-regime naturalization as a needed management tool [39], which was also recommended elsewhere [40,41].

2.3. Assemblage Assessments with Macrohabitat Considerations

Monahan et al. [42] assessed 23 hand-picked, wadeable stream sites across the USA by electrofishing. The highest fish-assemblage alpha and beta diversities were found in warmer, lowland rivers in Atlantic basins, where fish body sizes tended to be smaller. This paper highlighted the species-depauperate nature of USA Pacific basins, where colder headwaters favored larger-bodied salmonids [42]. Based on a 2554-site database from a probability survey, Hughes et al. [29] reported the same alpha and beta diversity patterns.

Heppell et al. [43] examined estuarine fin- and shellfishes in an Oregon estuary, focusing on abundance (CPUE) and biodiversity parameters in trawl samples over 3.5 decades. They found a shift from (i) English sole (Parophrys vetulus) and other demersal fishes to (ii) Dungeness crab (Cancer magister) and other epibenthic crustaceans. Sculpins (Cottidae) had also become more prevalent. Hence, there has been a shift away from pelagic fishes [43], as has been noted for other altered estuaries [44,45]. Although Heppell et al. [43] did not examine causal mechanisms, it is likely that both estuarine development (e.g., shoreline armoring, channel dredging, and a public marina) and the climate crisis were responsible.

2.4. Eurasian-Minnow Genetics, Hybridization, and Speciation

Valić et al. [46] performed genetic analyses on Illyrian chub (Squalius illyricus) and Zrmanja chub (Squalius zrmanjae) from the Krka River in Croatia, comparing them with sequences in GenBank. They found that S. zrmanjae had a nuclear region resembling Dalmatian rudd (Scardinius dergle), suggesting the transfer of genetic information across genera [46]. Notably, fish hybridization has occasionally been incorporated into MMIs [1] because habitat damage, pollution, and the climate crisis may limit interspecific-niche separation. This is of particular concern for rare, threatened, or endangered fish species [47,48].

Laskar et al. [49] examined morphology, genetics, and ranges of Osteobrama vigorsii and O. tikarpadaensis. Their paper helped resolve a long-term quandary regarding unusual distributions of Osteobrama species in India that should improve fish conservation efforts. Such an integrated approach with morphologic and molecular data should enhance the robustness of species assessments, with usefulness for fish conservation beyond India. Further consideration of life-history divergences [31,50,51] could help define evolutionarily significant units or distinct population segments for fish species.

3. Conclusions

This Special Issue collectively addressed biotic scales from (i) salmonid skin microbiomes to (ii) cyprinid genetics and ecology to (iii) assessment of ichthyoplankton and older fishes across freshwater and estuarine habitats. The focus was typically guild- or assemblage-oriented to better assess anthropogenic impacts, but it also included a single-species study [31] that examined geographic variation in catfish ecology in the face of climate crisis pressures. The genetic, microbiologic, habitat, trophic, and hydrologic ecology of fishes that were discussed should help us to better assess anthropogenic impacts in other contexts [1]. We hope this Special Issue provides a springboard for other aquatic ecologists to formulate more holistic, ecosystem-health assessments—especially by spatiotemporal planning—to minimize impacts to rare and migratory species.

Increasingly, ecologists must consider applied ecology, which is why most papers in our issue had a Conclusions section with management recommendations. Scientists can no longer shy away from environmental advocacy in a rapidly changing world, which requires us to make scientifically backed diagnoses [52] like what medical doctors must do to protect people and public health [2,5]. Hence, long-term biomonitoring with a stronger focus on aquatic biodiversity protection across biotic and spatiotemporal scales is needed [1,3,53]. That biomonitoring is required for rehabilitation projects to (i) verify that such efforts succeed [54,55], (ii) document ecosystem dynamics [56,57], and (iii) improve MMI and other impact-assessment analyses [1,14,58]. Clearly, true adaptive management is needed, which presently receives more lip service than effective implementation [2,5,6,55].